galaxy cruise

GALAXY CRUISE is a Citizen Astronomy (Citizen Science) project in which ordinary people and scientists come together to explore galaxies. In this spectacular space journey, you will decipher the history of galaxies from clues captured by a state-of-the-art instrument. Let's begin.

Features of GALAXY CRUISE

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Go on a journey to unlock the secrets of galaxies.  Add adventure to your everyday life.

Citizen scientists and AI take a cosmic cruise to discover 430,000 new galaxies

The discovery included 30,000 examples of the rarest known galaxy shape.

Citizen scientists and artificial intelligence have come together to discover a staggering 430,000 galaxies scattered across the universe. The massive haul includes 30,000 ring galaxies, which are considered to be the rarest of all possible galaxy shapes.

The discoveries represent the first results from the "GALAXY CRUISE" citizen science project. They were delivered by 10,000 volunteers who scoured through data collected with the Subaru Telescope . 

Subaru is an 8.2-meter, optical-infrared telescope located near the summit of the extinct volcano Maunakea on the island of Hawaii. This cutting-edge telescope collects a ton of incredible data — so much so that astronomers struggle to sift through it all to spot and classify the shapes of new galaxies. That is where the 10,000 citizen scientists came in, but even this vast army of volunteers needed a little assistance. Hence, they turned to AI for a helping hand.

Related: Speck of light glimpsed by Hubble is truly an enormous old galaxy, James Webb Space Telescope reveals

"Although AI classification takes less than one hour even for 700,000 galaxies, this work cannot be done without the data collected by GALAXY CRUISE over the past two years," team leader and Waseda University researcher Rhythm Shimakawa said in a statement. "We would like to thank all the citizen astronomers who participated in the project. I hope to see more collaborative outcomes in the future."

Hunting rare rings with AI

Galaxies come in a range of shapes and sizes ; often, these morphologies can reveal important galactic history. For instance, our own galaxy, the Milky Way , is a neat spiral galaxy . Spiral galaxies are named as such because of their long arms that wind out from a bright and dense central concentration of stars, gas and dust. This is the most common type of galaxy in the universe, accounting for approximately 70% to 80% of all galaxies

Far less common are ring galaxies , which account for no more than 1% to 3% of galaxies in the observable universe. In fact, in some estimates of ring galaxy populations, that number can fall as low as 0.1%.

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Ring galaxies get their name because they exhibit dense cores of ancient stars surrounded by distant rings of bright blue young stars. The first ring galaxy discovered by humanity was " Hoag's Object ," identified in 1950. 

To enable AI to make rapid classifications of galaxies, Shimakawa and colleagues first had to introduce it to catalogs of classifications made by humans. After familiarizing the AI with 20,000 galaxies classified by humans, they let it loose on 700,000 galaxies in the Subaru data.

The AI classified 400,000 galaxies as spiral galaxies and 30,000 of the galaxies as ring galaxies. This finally offers scientists a sample size of these rare galaxies that's large enough to start making a meaningful analyses of the realms.

The team determined that ring galaxies display characteristics that make them intermediates between spiral galaxies and what are known as elliptical galaxies, which have a less defined structure.

The current thinking is that they are created when two spiral galaxies slam together and merge. This violent process is known to wipe out the prominent features of spiral galaxies, particularly their distinctive spiral arms. The theory agrees with recent findings regarding ring galaxy formation, which suggest they are created in a special kind of collision. 

When one galaxy hits another head-on like a bullet, it's thought to cause pulses that radiate out from the galactic center of the target galaxy, spurring a ring structure.

— How do we know what the Milky Way looks like?

— The Milky Way galaxy may be a different shape than we thought

— Scientists reveal never-before-seen map of the Milky Way's central engine (image)

The team's findings not only broaden our understanding of an extremely rare galaxy type, but also demonstrate the power of AI to sort through vast catalogs of astronomical data.

The team's research was published in January in the journal Publications of the Astronomical Society of Japan.

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: [email protected].

Robert Lea is a science journalist in the U.K. whose articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University. Follow him on Twitter @sciencef1rst.

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A look aboard Disney’s $6,000 per stay, immersive Star Wars: Galactic Starcruiser hotel

A galaxy far, far away gets a whole lot closer.

By Chaim Gartenberg

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I walked into Star Wars: Galactic Starcruiser prepared to pretend. I was going to be Ch’aim Garteenbar, a reformed smuggler celebrating going straight with a luxurious cruise trip on the Halcyon, pride of Chandrila Star Line. 

Unfortunately, due to the compressed nature of the media preview, I spent most of my time aboard Disney’s biggest bet in years in my usual guise of “Chaim Gartenberg, reporter.” Part hotel, part LARP, part theme park, Star Wars: Galactic Starcruiser wants to be nothing less than the ultimate Star Wars experience, one that sucks you into a living, breathing sci-fi story unfolding around you — and for the price that Disney is placing on it, one that has a lot of expectations to live up to. 

To be extremely clear from the start: this is not a review of the full Galactic Starcruiser experience. The media preview I attended was a four-hour curated “sampler platter” of what guests will get over a two-day stay. I didn’t get to sleep at the hotel, and my time “on board” was focused on seeing the major set pieces and interactive components in group settings instead of the choice-based, smaller storylines that guests will get to build out over the full two-day trip. 

As such, this is not an assessment of whether the whole package is worth the eye-watering $4,809 starting price that Disney is charging for a Galactic Starcruiser stay. ( The exorbitant price tag was never far from my mind when I was touring the “ship,” though.)

While I can’t tell you everything about the minute-to-minute details of the branching, interactive storylines that Galactic Starcruiser is putting so much emphasis on, I did get to travel aboard the Halcyon and see some of the greatest hits and unique elements of the experience. 

Galactic Starcruiser, much like Galaxy’s Edge before it, isn’t just a creation for Disney theme parks. The modern desire to connect every element of media and storytelling together is a part of the Galactic Starcruiser experience, and Disney is already working to seed the luxury liner across decades of Star Wars history. 

The Halcyon plays a role in the ongoing High Republic multimedia storyline of books ; there’s already a comic book series that explains the backstory and shows iconic characters like Anakin Skywalker, Padme Amidala, and Lando Calrissian’s stays aboard the ship. There’s also the recently announced novel The Princess and the Scoundrel , which will reveal the untold story of Han and Leia’s honeymoon on the Halcyon. At the current rate of Galactic Starcruiser tie-ins, I’d put even odds on the ship appearing in an episode of a Disney Plus show before the year is out. 

Establishing the Halcyon as a key part of Star Wars lore isn’t just Disney not-so-subtly building hype and marketing for its pricey hotel, though. It’s a foundation for a key part of the sales pitch to guests. You don’t just get to live in a Star Wars - ish story for a day or two; your time aboard the Halcyon is part of the Star Wars saga — as much a part of canon as any of the movies, video games, or books. 

You’re participating in events but never really shaping them

That idea of living in an interactive hotel experience has seen Galactic Starcruiser draw a lot of comparisons to Westworld , the eponymous theme park from the HBO series. The two have a lot in common: a promise of total immersion in a fictional world (complete with factions and costumes), the idea that guests can forge their own stories within the setting, and the sky-high price tag that limits attendance to guests with exceptionally deep pockets. Galactic Starcruiser even has its own AI companions and roving robots (although they’re far less murderous than those in Westworld’s.)  

But Westworld’s impossible park promises a place where you can do anything or be anyone in the Wild West. Galactic Starcruiser puts more emphasis on “choice” over “agency.” A character can bring you to help sabotage the engine or rescue Chewbacca, but you won’t be the one making those decisions. Guests will have some control over which storylines they participate in and which side they’re on, but you’re still just a side character; you’re participating in events but never really shaping them.

The Galactic Starcruiser experience starts at a spaceport where you’ll ride a transport pod up to the Halcyon proper — the lines between “hotel” and “ride” blurring together from the start. 

Exiting the pod takes you to the Atrium, the Halcyon’s “lobby,” where you’ll run straight into guests and cast members alike milling around, ready to draw you into the plot. In quick succession, I met Gaya (the visiting galactic pop star), her manager Raithe Kole, an aspiring musician named Sandro Alimander (who, like me, was on his first trip off-planet), and the ship’s captain Riyola Keevan. Everyone had a story and, more importantly, wanted to try to involve me in it. Despite the elaborate costumes and setting, it still threw me off balance at first. I had to take a few minutes to figure out how I’d be fitting into the roleplaying. 

In some ways, the “ship” is a little too polished and clean. At a panel, Brad Schoeneberg, Disney’s vice president of brand merchandising, said it was inspired by “taking a look at the finer side of life in Star Wars, ” like the royal halls of Naboo, and (more oddly) the luxury yacht of crime lord Dryden Vos and the problematic halls of Canto Bight. It seemed like the visual language occasionally leaned more toward the gleaming ships of Star Trek than the lived-in style that’s the hallmark of Star Wars. There’s a glossy plastic look that never quite lets you forget you’re in what’s essentially a very large, very long theme park ride. 

The Halcyon is also smaller than you might expect. The main dining cabin is about the size of a medium-sized restaurant, while the atrium’s neon-soaked bar offers seating for a couple dozen people at most. (And only a single sabacc table!) There are “restricted” spaces that open up for story elements, like the cargo hold, brig, and engineering bay, but there are really only two or three main “public” spaces to spend time in outside of those plot moments.

The rooms I saw were pretty compact, too. There’s space for a bed, a slide-out table, and enclosed bunk beds built into one of the walls. It looks to have leaned too far into emulating the space-sensitive designs of an actual cruise ship. I’ve never stayed in a $2,000-per-night hotel room, but I would expect a lot more than this. 

No one will mistake Galactic Starcruiser for a traditional luxury hotel by Earth standards. There’s not much in the way of normal amenities — no pools, gyms, or spas, and virtually no natural light. (There is a single outdoor space, branded the “Climate Simulator” to avoid breaking character.) Sure, you wouldn’t go to Galactic Starcruiser for a regular hotel experience, but the price still feels like a miss.

Disney is putting plenty of emphasis on the food and drink aboard (both included in the ticket price, although specialty and alcoholic drinks are not). The presentation aims to go above and beyond a standard hotel menu. Much like Pandora or Galaxy’s Edge , food features unique shapes, colors, and textures to evoke a galaxy far, far away from our own, like colorful blue shrimp (allegedly sourced from Felucia) or black lava-rock bread evoking the volcanic world of Mustafar. 

At the Star Wars hotel, even the food has lore. 

Each trip on Galactic Starcruiser will follow the same broad story arc. Guests are passengers aboard the 275th anniversary trip of the luxury starcruiser, the Halcyon, which is emulating its original voyage from Chandrila to Batuu. Set in between The Last Jedi and The Rise of Skywalker , the ship runs afoul of the First Order — Star Wars drama ensues. 

Storylines, we’re told, will develop organically over the two-day trip, based on how players interact and react to cast members and the actions they take along the way. There are multiple branching paths, only some of which I got to take part in: smuggling a Resistance ally on board and sneaking into the engine room to reboot the ship’s systems — but there were other plots that appear at the finale event, like stealing access codes or unlocking a Jedi holocron, that weren’t in my (brief) journey. Someone else playing through a Resistance story might have seen more of those adventures, while a First Order-aligned guest would have had their own interactions to try to foil those plots. 

The whole thing culminates in an action-packed live stage show finale — it wouldn’t be Star Wars without a lightsaber duel (one which includes the spectacular-looking “real” lightsaber prop the company showed off a few months ago ). “You may all be watching the same big events, but you’re not watching the same things,” said Cory Rouse, a creative director for Walt Disney Imagineering, noting that a guest who sided with the Resistance will see payoffs that might not make sense to a scoundrel character, while a First Order ally will experience the finale in yet another light. 

Immersion is the name of the game here: each sign is written in both English and the fictional Aurebesh alphabet; every cast member, waiter, room service attendant, and bartender is in character the entire time; and elaborately costumed guests chat with you about their own lives to help draw you into the world. As you’d expect from a Disney park experience, there’s incredible attention to detail. All the doors on the ship are sliding, instead of on hinges, to better fit into the Star Wars universe.

But that level of theater is table stakes for Disney World. Galactic Starcruiser aims to take things a step further by actively looking to draw in guests. Visitors are encouraged to dress up in full Star Wars cosplay to inhabit the world, at least for a day or two. (Disney, of course, is happy to sell you Star Wars -themed attire if you don’t already have it.) And the key differentiator is the ability to participate in the adventure. 

The average Star Wars movie is about two hours long. Two seasons of The Mandalorian on Disney Plus will take you about half a day. Galactic Starcruiser aims for a far bigger scope: two days’ worth of Star Wars story. That means turning a lot of the ordinary parts of your day — including eating and sleeping — that you wouldn’t find in a movie into something more special and Star Wars -y. 

“[Movies] are a slice of life with all the boring cuts part out,” Rouse told The Verge . “Whenever we’re trying to build a world like this, we actually want you to have all of the extra — what we call the magical mundane,” turning those “boring parts” into something more. That might be a comment from your (in-character) server at the bar about the ongoings aboard the ship or a glimpse of a character like Rey gesturing at you from around a corner to help her with a mission.

“Do you want to play Star Wars?”

 “What they’re really asking you is ‘Do you want to play Star Wars?’” Rouse says. 

The word that Disney uses for most of Galactic Starcruiser’s more unique aspects is an “invitation.” As Ann Morrow Johnson, executive producer and creative director at Walt Disney Imagineering, noted in a press panel, “I think so much of this experience lives in these smaller subtler moments where these characters are existing, they’re living their lives, and they’re inviting you to come and play with them, if you want to.” 

At one point in our abridged trip, a few members of my group were pulled aside by a crew member to assist in a jailbreak for a captured Resistance member. Together, we hacked a terminal, passed the brig override code across a room in a short game of telephone, and unlocked the door so that they could escape. Presumably, I could have decided not to help if I wanted to aid the First Order instead. 

Galactic Starcruiser asks a lot of its guests. You’re basically diving into a giant improv exercise with dozens of strangers. And what you’ll get out of the experience is largely what you put in. In my short time there, it was hard to shake the feeling that I was just going through the motions for the particular story track my group was assigned. Maybe the smaller groups, added time, additional context, and more gradual pace of storytelling that the full experience has help make the process feel more organic. But that guided nature that slots you into specific roles is at least part of the point. 

Galactic Starcruiser asks a lot of its guests

According to Sara Thacher, a senior R&D Imagineer and the creative director behind all the branching narrative choices on board, Galactic Starcruiser is intended so that “you can show up and not have a character, not have to know any lines, not have to know anything about the story” and still have a good time. She described it almost like a traditional video game: players fall into a role from the game, and they can make choices but still have the framework and guidance they need to have fun. My short time on board didn’t really give me the chance to make those decisions for myself, however. 

Disney also wants to make sure guests are able to understand what choices they’re making: Thacher explained that an important goal is making sure that when you make a decision (to help someone or not), it’ll be clear what the broader outcome is: “We want you to know, ‘Yes, this means if I do this, I’m helping the Resistance,’ and presenting those choices as clearly as we can and really, as often as we can, as you as you encounter different pieces of the story, different pieces of the narrative.” In other words, if you’re looking to fight with the Resistance, you shouldn’t have to worry that talking to the wrong person will get you stuck for your whole trip as a First Order sympathizer. 

“Real choice means I have to understand the stakes,” Thacher adds. 

There are a few ride-like activities on the Halcyon, like the lightsaber training pod or the bridge, and the overall vibe here is more cruise ship than theme park — albeit a cruise ship that’s been ripped out of the Star Wars universe. 

The lightsaber training was (unsurprisingly) the best of them. Guided by a Saja (a Jedi-like follower of the Force), guests are given a lightsaber to spin and slash around to block blaster bolts. The effects and haptics here make the activity — not just light and sound, but you can actually feel the saber blocking each bolt as it flashes in the real world. 

The bridge area, meanwhile, is a more cooperative group activity in the vein of the Millennium Falcon: Smugglers Run ride at Galaxy’s Edge, with four different roles that guests rotate between. Bouncing across the different stations let me take a turn blocking asteroids, aligning the ship’s sensors by matching buttons and switches, blasting lasers, and more.

Those shipboard activities, in addition to offering entertainment in their own right, help serve as catalysts for drawing guests further into those story arcs. A bridge training exercise, for example, might end with helping the ship’s captain on a clandestine rescue mission in an asteroid belt. 

Guests will also journey down to the Galaxy’s Edge park in Disney World proper, where they’ll be able to ride the Millennium Falcon: Smugglers Run and Rise of the Resistance rides (Galactic Starcruiser guests will get to use the premium-purchase “ Lightning Lanes ” to avoid waiting in line, of course), along with accomplishing their own missions given to them from characters on the ship through the smartphone-based “Datapad” built into the Play Disney Parks app. Disney says those activities will also tie into guests’ individual, ongoing storylines when they come back aboard the ship. 

A franchise that you can not just love, but live in

The integration of narrative, traditional theme park rides, live entertainment, and a hotel stay make Galactic Starcruiser a blend of virtually every aspect of a Disney World trip all at once. It feels like the next step for the modern, all-encompassing pop culture machines that have come to dominate entertainment: a franchise that you can not just love, but live in. 

It’s also important to remember that Galactic Starcruiser is just the first of the company’s efforts here, one that will, like the parks it’s set in, presumably grow and change over time. And it’s easy to imagine that both Disney (and its competitors) will be thinking about how to bring this kind of first-hand fan experience to more properties in the future. 

That immersive experience comes at a very literal cost, though. When Disney first announced pricing, a two-night stay was quoted as starting at $4,809 for two adults, while a family of four (three adults, one child) was $5,999. That only includes the “basic” — relatively speaking — Galactic Starcruiser experience. Things like alcoholic or “specialty” drinks, outfits to better fit into the Star Wars universe you’re joining for your stay, and experiences like Savi’s Lightsaber Workshop on Baatu are extras you’ll have to budget in, too, additions to dive even deeper into the world. 

All this comes at a very literal cost

The end of our Galactic Starcruiser trip ended much in the same way that any day at Disney World does: a grand fireworks show celebrating a successful voyage. The explosions are bright and colorful, with shapes and designs far beyond anything terrestrial fireworks could do. But they’re also fake: they’re digital illusions playing on the massive “viewport” screens on the bridge, masked by the soaring John Williams score and the heightened reality you’re in. 

Photography by Chaim Gartenberg / The Verge

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First scientific results from Galaxy Cruise

by Subaru Telescope

Galaxy Cruise, a citizen science project led by the National Astronomical Observatory of Japan (NAOJ), has been sailing the cosmic ocean with citizen astronomers to uncover the secrets of galaxies since 2019.

Using the deep, high-quality images from the Subaru Telescope combined with high-accuracy classifications of galaxies provided by the citizen astronomers, professional researchers unambiguously confirmed that galaxies become more active when they collide and merge with other galaxies. This result is reported in the first scientific paper from Galaxy Cruise. The citizen astronomers' classifications are made available to the public so that astronomers from all over the world can use them to make new discoveries.

This work was published online in Publications of the Astronomical Society of Japan on September 26, 2023, titled " Galaxy Cruise: Deep Insights into Interacting Galaxies in the Local Universe ."

The universe is filled with a wide variety of galaxies; some galaxies are red elliptical galaxies, while others are blue spiral galaxies. There are also galaxies without well-defined shapes. This diversity is thought to arise from collisions and mergers between galaxies over cosmic timescales. However, the exact roles of collisions and mergers have remained poorly understood because interacting galaxies are rare objects and are thus difficult to find.

To overcome this difficulty, Galaxy Cruise called for help from citizen astronomers to identify interacting galaxies from the deep images taken with Hyper Suprime-Cam (HSC) installed on the Subaru Telescope.

Citizen astronomers are by no means professional astronomers, but can they classify galaxies? Yes, they can! Professional astronomers show them how. Citizen astronomers are asked to go through a training course to understand the fundamentals of galaxy morphology. Once they pass the course, they get a boarding pass for Galaxy Cruise.

Many citizen astronomers got onboard and were captivated by the diversity of galaxies in the universe. About 10,000 citizen astronomers explored the universe and made more than 2 million classifications in the first 2.5 years of Galaxy Cruise. Such a large number of classifications would not have been possible by professional astronomers alone.

Dr. Masayuki Tanaka, the "Captain" of Galaxy Cruise, carefully analyzed the citizen scientists' classifications and found that the citizen astronomers classified galaxies very well. Figure 2 illustrates the accuracy. The quality of the HSC images is essential for the high classification accuracy; there are many galaxies that were classified as elliptical galaxies in previous studies, but they turn out to exhibit clear spiral arms in the deeper HSC images. "The classification accuracy of Galaxy Cruise surpasses previous studies," says Tanaka.

The same applies to interacting galaxies, which are the focus of Galaxy Cruise. When galaxies collide and merge, they often show distorted shapes with characteristic features around them such as tidal tails. These features are often diffuse and faint and can easily be missed. However, thanks to the high sensitivity and high angular resolution of the HSC images, Galaxy Cruise successfully captured these faint features.

The citizen astronomers discovered that many of the 'normal' (i.e., non-interacting) galaxies reported in the previous studies actually exhibit signs of interaction. In addition, citizen astronomers identified galaxies that are currently undergoing violent mergers.

Figure 3 showcases these violent mergers. They all show significantly distorted shapes with very complex structures. Such violent mergers are extremely rare and a statistical sample of such galaxies illustrates the power of visual classifications by a large number of citizen astronomers.

The sample of interacting galaxies revealed that these galaxies show an enhanced level of star formation activity (Note 1) compared to normal galaxies. Furthermore, super massive black holes are also found to be more active. Interestingly, this activity is most significantly enhanced in violent mergers like those shown in Figure 3.

It is likely that the final coalescence of a merger event is the moment when the internal activity of galaxies is most strongly enhanced. These interesting results were published in a scientific paper. This is the first paper from Galaxy Cruise and is a milestone not just for astronomers but also the participating citizen astronomers.

"There have been a lot of efforts trying to understand the star formation and black hole activities of merging galaxies," says Tanaka. "However, researchers have often reached contradicting results. This is likely due to the difficulties in identifying merging galaxies, differences in the definition of mergers, differences in the way the galaxies are analyzed, etc. Galaxy Cruise's approach to the problem is the classic visual classification."

"It is a time consuming but powerful method to identify mergers. Combined with the high-quality HSC images, we could construct a better sample of mergers than before and it led us to confirm unambiguously that mergers enhance the internal activities of galaxies. This is an extremely exciting result and it would not have been possible without the participation of so many citizen astronomers."

Galaxy Cruise's classification catalog has been released to the public together with the publication of the paper. The high-quality classifications will be further exploited by professional astronomers from all over the world. The catalog will hopefully lead them to new discoveries.

"People may think that scientific work is only for professional researchers. That is not necessarily the case. Public citizens can certainly contribute as Galaxy Cruise demonstrated. Galaxy Cruise is still on its continuing voyage. Why not join us? I am looking forward to welcoming you on board and solving the mysteries of galaxies together," Captain Tanaka concludes.

Provided by Subaru Telescope

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National Astronomical Observatory of Japan

You are an astronomer —galaxy cruise season 2 has launched.

April 18, 2022 | Topics

GALAXY CRUISE is a project in which citizen volunteers participate in galaxy research while exploring the vast cosmic images captured by the Subaru Telescope. As Season 2 of this exciting project, Deep Quest has just launched to delve even deeper into the mysteries of galaxies.

GALAXY CRUISE started in 2019, aiming to unravel the mysteries surrounding the evolution and diversity of galaxies through cooperation between citizens and scientists. Galaxies exhibit a remarkable morphological diversity, from elliptical galaxies with ill-defined structures to spiral galaxies with disks and spiral arms. Galactic mergers and mutual interactions may hold clues to understanding the origins of this diversity. During Season 1, Citizen Astronomers classified more than 20,000 galaxies found in the second public data release from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP), an extensive survey with Hyper Suprime-Cam (HSC) mounted on the Subaru Telescope. No less than 50 Citizen Astronomers classified all the galaxies, providing us with sufficient classification results for statistical analysis.

Equipped with an 8.2-m primary mirror, the Subaru Telescope has high resolution and light-gathering power which allow us to observe faint structures in celestial objects. The scientific analysis of the Season 1 classifications has found spiral structures in galaxies that were once identified as ellipticals, and signs of interactions and mergers in galaxies that were once thought to be non-interacting.

Season 2 focuses on classifying fainter galaxies captured in the same images as were used in Season 1. The classification results for fainter, smaller galaxies within the same distance range will make even more detailed analysis possible. The title “Deep Quest” is meant to reflect its ambition to delve deeper into the mysteries of galaxies. “Thankfully in Season 1, many Citizen Astronomers classified galaxies with high accuracy,” says Captain Masayuki Tanaka, an associate professor at Subaru Telescope. “As a result, we were able to analyze classification results that would not have been collected by researchers alone, and we found that previous studies need to be followed up with studies based on ‘Subaru Telescope quality.’ As in the previous season, we hope you will help our cutting-edge galaxy research while exploring the vastness of the cosmic ocean.”

GALAXY CRUISE started as a citizen science project of NAOJ on November 1, 2019, in Japanese, and the English version opened on February 19, 2020. As of April 1, 2022, more than two years after the project started, 9,742 people (of which 6,854 are from Japan) from 92 countries and regions have signed up, and the total classification results have exceeded 2.5 million. Citizen science is scientific work undertaken by members of the general public (citizens), often in collaboration with professional scientists.

GALAXY CRUISE has been generating new discoveries through scientific analysis of citizens’ classifications, and its applications have now expanded to include an educational program to provide research experience for university students who will be the scientists of the future. What kind of journey will await us in Deep Quest? We cordially invite you to come on board and start your voyage with professional scientists.

GALAXY CRUIS Website

Related Links

• You are an Astronomer! —GALAXY CRUISE Season 2 Has Launched(Subaru Telescope)
• GALAXY CRUISE 2022—A New Journey (GALAXY CRUISE)
• NAOJ Researchers Win 2021 MEXT Commendations for Science and Technology (June 16, 2021)
• GALAXY CRUISE—Your Galactic Journey as a Citizen Scientist (February 19, 2020)
• Amazing Universe Captured with the Subaru Telescope! “HSC Viewer” Released to the Public (March 8, 2018)
• Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP)

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Stellar Discoveries: GALAXY CRUISE’s First Dive Into Colliding Galaxies

By National Astronomical Observatory of Japan November 21, 2023

Galaxies identified as showing strong signs of interactions in GALAXY CRUISE. Credit: NAOJ

GALAXY CRUISE, with the help of 10,000 citizen astronomers, discovers that galaxies’ star formation rate surges during collisions, as highlighted in their first scientific paper.

The citizen science project GALAXY CRUISE has yielded its first scientific paper. The results show unambiguously that the star formation rate is enhanced in colliding galaxies. This research was made possible by the approximately 10,000 citizen astronomers who helped classify galaxies in data from the Subaru Telescope.

Astronomy survey programs are producing more high-resolution data than astronomers can analyze. Great strides are being made in computerized data processing, but for some tasks, the human eye is still the most accurate measurement device. To process the data, professional astronomers led by Masayuki Tanaka in the GALAXY CRUISE project ask private citizens to look at the data and help classify the features of galaxies.

Based on the more than 2 million classifications by about 10,000 citizen astronomers over the course of two and a half years, GALAXY CRUISE has published its first scientific paper.

The presented results show that when galaxies collide and merge with each other, it increases the rate at which new stars are formed in the galaxies.

The GALAXY CRUISE project is continuing to see what other secrets can be found in the data with the help of citizen astronomers. For more details, please refer to the GALAXY CRUISE website.

For more on this research and GALAXY CRUISE, see Astronomers and Citizen Sleuths Uncover Galactic Secrets .

Reference: “Galaxy Cruise: Deep Insights into Interacting Galaxies in the Local Universe” by Masayuki Tanaka, Michitaro Koike, Sei’ichiro Naito, Junko Shibata, Kumiko Usuda-Sato, Hitoshi Yamaoka, Makoto Ando, Kei Ito, Umi Kobayashi, Yutaro Kofuji, Atsuki Kuwata, Suzuka Nakano, Rhythm Shimakawa, Ken-ichi Tadaki, Suguru Takebayashi, Chie Tsuchiya, Tomofumi Umemoto and Connor Bottrell, 26 September 2023,  Publications of the Astronomical Society of Japan . DOI: 10.1093/pasj/psad055

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Article Contents

1 introduction, 2 galaxy cruise, 3 classification accuracy, 4 properties of galaxies in the local universe revisited, acknowledgments.

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Galaxy Cruise: Deep Insights into Interacting Galaxies in the Local Universe †

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Masayuki Tanaka, Michitaro Koike, Sei’ichiro Naito, Junko Shibata, Kumiko Usuda-Sato, Hitoshi Yamaoka, Makoto Ando, Kei Ito, Umi Kobayashi, Yutaro Kofuji, Atsuki Kuwata, Suzuka Nakano, Rhythm Shimakawa, Ken-ichi Tadaki, Suguru Takebayashi, Chie Tsuchiya, Tomofumi Umemoto, Connor Bottrell, Galaxy Cruise: Deep Insights into Interacting Galaxies in the Local Universe, Publications of the Astronomical Society of Japan , Volume 75, Issue 5, October 2023, Pages 986–1010, https://doi.org/10.1093/pasj/psad055

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We present the first results from GALAXY CRUISE, a community (or citizen) science project based on data from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). The current paradigm of galaxy evolution suggests that galaxies grow hierarchically via mergers, but our observational understanding of the role of mergers is still limited. The data from HSC-SSP are ideally suited to improve our understanding with improved identifications of interacting galaxies thanks to the superb depth and image quality of HSC-SSP. We launched a community science project, GALAXY CRUISE, in 2019 and have collected over two million independent classifications of 20686 galaxies at z  < 0.2. We first characterize the accuracy of the participants’ classifications and demonstrate that it surpasses previous studies based on shallower imaging data. We then investigate various aspects of interacting galaxies in detail. We show that there is a clear sign of enhanced activities of super-massive black holes and star formation in interacting galaxies compared to those in isolated galaxies. The enhancement seems particularly strong for galaxies undergoing violent mergers. We also show that the mass growth rate inferred from our results is roughly consistent with the observed evolution of the stellar mass function. The second season of GALAXY CRUISE is currently underway and we conclude with future prospects. We make the morphological classification catalog used in this paper publicly available at the GALAXY CRUISE website, which will be particularly useful for machine-learning applications.

Astronomy is one of the research fields where not only professional researchers but also the general public have made significant contributions to advance the field. An old example can be found back in the 18th century when Edmond Halley asked “the Curious” to observe the total solar eclipse over England (Pasachoff 1999 ). Astronomy has a long tradition of active contributions of the general public and amateur astronomers, and new discoveries of interesting objects such as comets and supernovae are often reported even today, some of which are followed up by professional astronomers with a larger telescope for detailed studies. This teamwork between non-professional and professional researchers has been extremely fruitful to improve our understanding of the Universe.

A new area of such collaboration emerged with the advent of large sky surveys, which observe billions of celestial objects in a large area of the sky. While objects from these surveys are detected and measured in an automated fashion, not all properties of the objects can be fully captured and cataloged. Galaxy morphology is one such property; there is no easy measurement that fully describes the entire variation of galaxy morphology. Morphology has traditionally been classified by human eyes as they are good at recognizing characteristic patterns in images such as spiral arms. A number of automated methods have been proposed, such as concentration index (Morgan 1958 ; Doi et al. 1993 ; Abraham et al. 1994 ), Sersic index (Sérsic 1968 ), Gini coefficient (Abraham et al. 2003 ), etc., but human classifications still remain highly complementary to these automated classifications.

Galaxy Zoo (Lintott et al. 2008 ) is the first large-scale citizen science (hereafter community science) project based on the imaging data from the Sloan Digital Sky Survey (SDSS; York et al. 2000 ). Participants classify morphologies of objects in the SDSS image and a large number of independent classifications have been collected for nearly a million objects. There are many scientific papers based on the Galaxy Zoo’s classifications published to date. Galaxy Zoo is undoubtedly the most successful community science project in astronomy and the successor and spin-off projects such as Galaxy Zoo 2 (Willett et al. 2013 ) have also been successful.

The Galaxy Zoo’s classifications (Lintott et al. 2008 ) are fundamentally limited by the imaging quality of SDSS. While SDSS delivered the state-of-the-art imaging data at that time, recent imaging surveys such as the Dark Energy Survey (DES; The Dark Energy Survey Collaboration 2005 ), PanSTARRS1 (PS1; Chambers et al. 2016 ), and Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP; Aihara et al. 2018a ) yield deeper images, which help improve the classification accuracy. Among these surveys, HSC-SSP is particularly interesting due to its superb image quality and depth; the median seeing in the i band is |${0{^{\prime \prime}_{.}}6}$| and the images reach 26–27th magnitude, i.e., 2–4 magnitudes deeper than SDSS, DES, and PS1. The depth is particularly important to detect faint spiral arms as well as diffuse tidal features, as we demonstrate in this paper.

Based on the imaging data from HSC-SSP, we have launched GALAXY CRUISE, the first Japanese community science project in astronomy. 1 In order to exploit the superb imaging data fully, we put an emphasis on interacting galaxies. In the bottom-up galaxy formation scenario, we expect that galaxy–galaxy mergers play a fundamental role in the evolution of galaxies over the cosmic time. However, our understanding of mergers is still limited; frequency of mergers, triggering of starbursts, and active galactic nuclei (AGNs) still remain unclear. This is largely due to the difficulty of identifying interacting/merging galaxies. A traditional way to identify mergers is to look for tidal features around galaxies, but these features are typically very faint and deep imaging data are essential for identifying them (e.g., Lotz et al. 2008 ; Tal et al. 2009 ; Duc et al. 2015 ; Bottrell et al. 2019 ; Wilkinson et al. 2022 ). To avoid this difficulty, close pairs are also often used to estimate the merger rate (Patton et al. 2000 , 2002 ; Lin et al. 2004 ). However, close pairs have their own difficulties, such as contamination of line-of-sight pairs that are not physically bound and a lack of sensitivity to post-merger phases. We focus on the former approach to exploit the imaging depth of HSC-SSP fully in this paper and present the first results from GALAXY CRUISE.

The paper is organized as follows. We introduce GALAXY CRUISE and discuss classification statistics in section  2 . We then characterize the classification accuracy in section  3 . Based on the participants’ classifications, we make comparisons to previous community science projects and also examine properties of interacting galaxies in section  4 . Finally, the paper is summarized in section  5 . Unless otherwise stated, we use the AB magnitude system (Oke & Gunn 1983 ) and adopt the flat cosmology with |$\rm H_0=70\:$| km s −1  Mpc −1 , |$\rm \Omega _M=0.3$|⁠ , and |$\Omega _\Lambda =0.7$|⁠ .

2.1 Motivation

We focus on interacting galaxies to address some of the outstanding questions about galaxy mergers as discussed in the previous section. As interacting galaxies are a rare population, we need to inspect many galaxies to gain sufficient statistics to study their properties. However, the visual inspection of tens of thousands of galaxies is difficult for professional astronomers to perform, as astronomers are a rare population. This is where the interested general public can significantly contribute, as Galaxy Zoo demonstrated. We follow the same approach and ask participants to classify galaxies and find interacting galaxies.

A community science project is not just a science project, but also has a major aspect of public engagement. We put an emphasis on this aspect in GALAXY CRUISE and implemented a lot of gamification factors to motivate and entertain participants as they classify objects. As part of this, we made the theme of the classification site a cruise ship sailing across the cosmic ocean, hence the name GALAXY CRUISE. Participants collect passport stamps and souvenirs as they make progress. There are also seasonal campaigns to make, e.g., a snowman as they classify. We encourage participants to not just classify targets but also freely explore the vast Universe. To this end, we do not to show postage stamps of galaxies and instead show the entire sky observed by HSC-SSP as a contiguous image. The contiguous image is essential also for identifications of tidal features; we do not know a priori where tidal features are and some of them may be located far away from the central galaxy. A fixed-scale postage-stamp may miss such features, but the contiguous image does not. A screenshot of our classification site is shown in figure  1 .

GALAXY CRUISE’s classification screen. Participants classify the target at the center, and then “sail” to the next target. Passport stamps, souvenirs, and other features are available to motivate the participants.

As mentioned above, we use publicly available images from HSC-SSP. HSC-SSP is a wide and deep survey of the sky using HSC installed at the Subaru Telescope. It has three survey layers. The Wide survey covers ∼1100 square degrees of the sky located mostly around the equator in five broad-band filters ( grizy ) down to r ∼ 26.5 for point sources at 5σ. The Deep survey has four fields separated roughly equally in RA, so that one of them is observable at any time. The Deep survey typically goes a magnitude deeper than Wide. Finally, the UltraDeep survey is two fields, Cosmic Evolution Survey (COSMOS) and Subaru/XMM-Newton Deep Survey (SXDS), reaching down to ∼28th mag. Further details of the survey can be found in Aihara et al. ( 2018b ). The HSC-SSP data are routinely made public and we use multi-band images from HSC-SSP Public Data Release 2 (PDR2; Aihara et al. 2019 ), which was the latest release when our project started. Most of our targets are from the Wide survey, but we also include objects from the Deep and UltraDeep surveys allowing for duplication, so that we can characterize the dependence on the depth in our classifications (see the next section for details). Table 2 of Aihara et al. ( 2019 ) summarizes the quality of the imaging data. We combine the g, r , and i bands to generate color images of the sky using the Lupton et al. ( 2004 ) scheme. Participants can adjust levels so that bright/faint parts of targets can be easily seen. This feature is particularly useful for identifying interaction features, which are often diffuse and extended. Also, single-filter black/white images are available for detailed inspections.

While our primary objective is to identify interacting galaxies, we are also interested in the morphology of normal (non-interacting) galaxies because the depth and quality of HSC may bring a difference in our basic understanding of galaxy morphology in the local Universe. We choose to make two-step classifications; the first step is to classify morphology of galaxies such as elliptical and spiral, and the second is to look for interaction features. There are multiple morphology classification schemes. Some are rather complicated and difficult, while others are more straightforward. How detailed the participants can make classifications and with what accuracy is not a trivial question to answer.

In order to address the question, we conducted public experiments at the National Museum of Emerging Science and Innovation (Miraikan) in Tokyo. We displayed a large-format poster of a contiguous region of the sky taken by HSC-SSP. We made an introductory lecture about galaxy morphology to participants and asked them to classify target galaxies marked on the poster. We also talked to them directly to see how they recognize various features of galaxies.

Lessons learned from the experiment include: (1) the general public can classify bright galaxies reasonably well, but (2) faint galaxies (for which basic classifications can still be made by professional astronomers) are very difficult, and (3) subtle differences such as presence of weak bars, elliptical vs. S0, etc., are difficult to recognize. Following these lessons, we choose to focus on bright galaxies and adopt a simple classification scheme, which we detail in the next subsection. Another lesson learned from the experiment is that edge-on spirals tend to be classified as ellipticals. While we made an attempt to make the difference clear in the tutorial, this tendency is still observed in our classifications in GALAXY CRUISE. The introductory lecture used in the experiment turned into training courses for first-time participants at the GALAXY CRUISE website. Overall, the public experiment was an essential piece of our project.

2.2 Classification scheme

Figure  2 shows our classification flow. The first question is whether a galaxy is an elliptical galaxy or a spiral galaxy. As described earlier, subtle differences between ellipticals and S0s and bar structure are difficult for the general public to distinguish. We thus choose to adopt the simplified classification scheme. Some interacting galaxies show significantly distorted shapes and their original morphology can be difficult to infer. In such a case, the participant can choose “not sure.” When a target galaxy is too close to a nearby bright star and a fair classification is difficult, the participant can choose “not sure” as well.

GALAXY CRUISE’s classification scheme. The first question is whether a galaxy is spiral or elliptical. The second question asks if the galaxy is interacting or not. If yes, the participant can choose observed feature(s).

The second question is whether there is a sign of interaction or not. Interacting galaxies exhibit a variety of features, but we define four typical features; (1) tidal tail/stream, (2) distorted shape, (3) fan/shell structure, and (4) ring structure. The first feature is perhaps the most prominent feature seen around interacting galaxies; leading/trailing streams are formed by stars escaping from an infalling galaxy mostly through the Lagrange points L 1 and L 2 due to tidal disruption (Eyre & Binney 2011 ). A distorted shape is also a common feature and is also caused by tidal forces. A fan/shell feature is likely a special case of collision with a small impact parameter (i.e., low angular momentum), which results in two-sided caustics with large opening angles (Pop et al. 2018 ). Finally, the ring structure can be formed by multiple processes including a secular process, but galaxy–galaxy interactions are a strong theory for the origin of ring galaxies (e.g., Elagali et al. 2018 ) and we interpret the ring structure as a possible sign of interaction.

One can choose multiple interaction features for a given object. This is because multiple features are often seen (especially distortion and tail) and also because some of the features are not easily distinguishable (e.g., arc-like tail vs. fan). As we discuss below, tail and distortion features are the most common features and ring and fan features are less often observed.

Before a participant joins GALAXY CRUISE and starts to make classifications, we ask them to go over an online introductory course, which is based on the tutorial from the public experiment as mentioned above, to understand our classification scheme. The online course is a three-step tutorial. We first explain the primary differences between elliptical and spiral galaxies, such as the presence of spiral arms and disk structure. We then show that galaxies are normally symmetric in shape and deviations from the symmetry are often an indication of interaction. Finally, we introduce the four typical interaction features discussed above. We ask several questions to the participant at each step to make sure that the participant understands the scheme. These three steps are identical to the questions shown in figure  2 . In other words, we explain each question in the tutorial. After the tutorial, the participant gets a “boarding pass” to join GALAXY CRUISE. For further details, the reader is referred to the GALAXY CRUISE website.

2.3 Target galaxies

GALAXY CRUISE is based on deep imaging data from HSC-SSP PDR2 (Aihara et al. 2019 ), in which the global sky subtraction was applied to keep extended wings of bright objects. This is an ideal feature for our purpose as interaction features are often diffuse and extended.

The target galaxies for classifications are drawn from spectroscopic data from the SDSS (York et al. 2000 ) and the Galaxy And Mass Assembly (GAMA) survey (Driver et al. 2009 ). The reasons why we draw targets from the spectrscopic data include (1) they are bright objects for which the participants can make good classifications (see subsection 2.1 ), and (2) the distance information allows us to infer physical properties of galaxies, such as star formation rates (SFRs) from emission line fluxes, accurately, and (3) the spectra allow us to identify AGNs from emission line intensity ratios. The SDSS main galaxy sample (Strauss et al. 2002 ) is primarily used here, but any objects with spectroscopic redshifts from Data Release 15 (Aguado et al. 2019 ) located at z  < 0.2 are included. We imposed this redshift cut to make sure that we focus on nearby galaxies for which we expect to have sufficient spatial resolution to classify. We supplement the SDSS redshifts with those from GAMA DR2 (Liske et al. 2015 ) to achieve a higher sampling rate. The same redshift cut of z  < 0.2 is imposed here as well. We then apply a magnitude cut to the cModel photometry in the z -band ( z cmodel  < 17.0) to the entire sample to ensure that we focus on very bright galaxies.

The magnitude cut was applied using the HSC photometry, which later turned out not to be the best choice; due to the high spatial resolution of HSC, bright galaxies, especially face-on spiral galaxies, are occasionally deblended into multiple pieces (Bosch et al. 2018 ; Aihara et al. 2019 ). This resulted in reduced brightness of the central galaxy, and a fraction of bright galaxies were excluded from the target sample by the magnitude cut. The same problem occurred in SDSS as well but to a lesser extent. A comparison between HSC and SDSS photometry indicates that this effect is not significant for most sources, but about |$10\%$| of the sources have fainter HSC photometry by more than 1 mag. In order to mitigate the problem, the GALAXY CRUISE second season has been running since 2022 April. The second season is an extension of the first season, but we target fainter galaxies without any magnitude cut including those that should have been targeted in the first season. They are still bright galaxies with spectroscopic redshifts, following the lesson learned from the public experiment described in subsection 2.1 . We note that all the photometry used in what follows is from SDSS.

The targets are drawn from SDSS (18960 galaxies) and GAMA (1726 galaxies). Among them, 20116 galaxies are in the Wide layer, and 1410 galaxies are in the D/UD layer. As three of the four HSC-SSP D/UD fields are embedded in the Wide fields, 840 galaxies are duplicated and classified twice; once at the D/UD depth and once at the Wide depth. We evaluate how classifications change depending on the image depth in subsection 3.1.

2.4 Combined classifications

In this paper, we base our analysis on the classifications collected during the first season between 2019 November and 2022 April. Due to the way classifications were made, a small number of participants classified the same objects multiple times. We use only the last classification. Figure  3 shows the number of classifications made for each object. The median of the distribution is 83, which is sufficient for statistical classifications of objects. We define a probability that an object is a spiral galaxy as

where N total is the total number of classifications made for that object, and N spiral is the number of spiral classifications among them. If |$P(\mathit {spiral})=1$|⁠ , everyone agrees that an object is a spiral galaxy. If |$P(\mathit {spiral})=0$| instead, then everyone agrees that it is an elliptical galaxy. Of course, there are other types of galaxies in the Universe, such as S0 galaxies, but we adopt this simplified classification scheme as discussed in subsection  2.2 . Strictly speaking, |$P(\mathit {spiral})$| is not a probability but just a fraction of classifications. However, we later discuss the fraction of spiral galaxies among our sample. In order to reduce confusion about which fraction we are discussing, we refer to the fraction of spiral classifications by the participants as the “spiral probability” or |$P(\mathit {spiral})$|⁠ . In the same manner, we define |$P(\mathit {int.})$|⁠ , which indicates the fraction of people who vote for interaction in the second question.

Distribution of the number of classifications for each object. The open histogram is the original classifications, while the cross-hatched one is after the cleaning of bad classifications (see text for details). The median of the distribution is 83 for the original and 74 for the cleaned catalogs, respectively.

We first merge all of the individual classifications and compute |$P(\mathit {spiral})$|⁠ . While most participants carefully classify objects, there are a small number of people whose classifications are less accurate. To make a reliable morphological catalog, we make an attempt to exclude those less-accurate participants. To this aim, we focus on galaxies with obvious morphologies, i.e., galaxies for which the vast majority of people agree on their morphological types. To be specific, we select objects with |$P(\mathit {spiral})<0.05$| or |$P(\mathit {spiral})>0.95$|⁠ . These are clearly either elliptical or spiral. Such objects comprise about |$10\%$| of the entire sample. We then compile a list of participants whose classifications fall in the minority (i.e., likely incorrect). We define the probability that a participant makes an inaccurate classification as

where |$N(\mathit {obvious})$| is the number of objects with obvious morphology that a participant classified, and |$N(\mathit {bad})$| is a subset of |$N(\mathit {obvious})$| and the number of classifications made by the participant that fall in the minority. Figure  4 shows |$P(\mathit {bad})$| against the total number of classifications that a participant made. As can be seen, most participants make good classifications with low |$P(\mathit {bad})$|⁠ , but there are people whose classifications are less accurate. We choose to exclude classifications by participants with |$P(\mathit {bad})>0.1$| in this work. This excludes 276530 classifications out of the total of 2431455 classifications ( ⁠|$11\%$|⁠ ). We have confirmed that our results do not change if we include them. We note that Galaxy Zoo adopted a similar approach; they down-weighted participants who consistently disagreed with the majority. Note as well that there is a group of participants at N classification ∼ 1000 in figure  4 . This is likely due to classification campaigns we have promoted, during which 1000 classifications give participants a special image.

|$P(\mathit {bad})$| plotted against the total number of classifications for each participant. Participants with |$P(\mathit {bad})>0.1$| (the red points) are excluded from the main analyses. The other participants shown in black are included.

The cross-hatched histogram in figure  3 shows the number of classifications for each object after the less-accurate participants are excluded. The median number of classification is reduced from 83 to 74, but it is still sufficient for statistical analyses in this paper. We use |$P(\mathit {spiral})$| and |$P(\mathit {int.})$| after excluding these inaccurate classifiers in what follows.

For the first and second questions, we include the “not sure” option as shown in figure  2 . It is intended to flag objects that are difficult to classify because the target is very strongly disturbed. We find that the majority ( ⁠|$92\%$|⁠ ) of objects have |$P(\mathit {not\_sure})<0.1$| for the first question. Careful inspections of individual cases suggest that most participants did not choose “not sure” even when the target is strongly disturbed. For instance, objects with |$P(\mathit {not\_sure})\sim 0.3$| are often undergoing violent interactions, but |$\sim 70\%$| of the participants classified them into elliptical/spiral. Also, it seems that a small fraction of people used the “not sure” option for simply difficult cases (e.g., galaxies that appear small and hard to classify into ellitical or spiral). This was not our original intention for this option. The trend is similar for the second question; many of the objects with |$P(\mathit {not\_sure})>0.1$| are simply difficult cases such as targets in dense clusters, where the intracluster light is clearly visible.

As some fraction of participants used the “not sure” option in the way that we originally did not intend, we choose to exclude “not sure” votes when we compute |$P(\mathit {spiral})$| and |$P(\mathit {int.})$| [i.e., N total in equation ( 1 ) is equal to N spiral + N elliptical ] because it is the least harmful way to handle them. We have confirmed that our conclusions remain the same if we include them (i.e., |$N_{\rm total}=N_{\rm spiral} + N_{\rm elliptical} + N_{\rm not\_sure}$|⁠ ). We should emphasize, however, that the “not sure” votes are not useless; they actually turn out to be quite useful to identify violent mergers, as we demonstrate below.

Visual classifications of galaxy morphology are dependent on the imaging depth, e.g., faint arms of a spiral galaxy may not be clearly identified in a shallow image. Diffuse tidal streams may be missed for the same reason. There is also redshift dependence, as a faint feature is harder to identify at higher redshift. Furthermore, there may be yet another effect arising from systematic biases in the participants’ classifications. We make a few tests in this section to characterize the depth and redshift dependence as well as classification biases.

3.1 Depth dependence

We first focus on the dependence on the imaging depth. As we noted above, a small number of the targets are classified in both the Wide and D/UD layers. This is done exactly for the purpose of the depth-dependence characterization; as the D/UD layer is deeper than the Wide layer (the typical exposure time is about five times longer; Aihara et al. 2019 ), we can make a direct comparison between them for the same set of objects.

Figure  5 makes this comparison and illustrates the role of depth at fixed resolution by comparing the Wide layer with the D/UD layer. |$P(\mathit {spiral})$| in the left-hand panel shows that, while there is a good agreement at |$P(\mathit {spiral})\sim 0$| and 1, the different depths give a slightly different |$P(\mathit {spiral})$| at intermediate |$P(\mathit {spiral})$|⁠ . We interpret this trend as spiral galaxies being more robustly identified in the deeper images. As we demonstrate below, GALAXY CRUISE finds a higher fraction of spiral galaxies compared to previous projects because of the much improved depth and quality. That is, spiral features can be more securely identified in deeper and sharper images. This explains the trend seen in figure  5 ; |$P(\mathit {spiral})$| tends to be larger in D/UD than in Wide for ambiguous cases. The depth difference is less important when a target galaxy is obviously elliptical or spiral, and |$P(\mathit {spiral})$| agrees well at 0 and 1.

Left: Comparison of |$P(\mathit {spiral})$| between the Wide and Deep/UltraDeep layers for the same set of objects. A point is an object and the associated error bars indicate the statistical uncertainty. The dashed line is the one-to-one relationship, while the dotted curve is the median of the distribution running over |$P(\mathit {spiral})$| for Wide. Right: As in the left-hand figure but for |$P(\mathit {int.})$|⁠ .

The right-hand panel, which shows |$P(\mathit {int.})$| for D/UD and Wide, can be explained in the same way. As interaction features are often faint and diffuse, deeper images reveal a larger fraction of interacting galaxies. The dotted curve in both panels show the running median of the distribution and we use this curve to statistically correct |$P(\mathit {spiral})$| and |$P(\mathit {int.})$| from Wide to the D/UD depth. The correction would be larger if we had deeper images than D/UD (i.e., we would identify more interacting features if we could go deeper), but the dotted curve in the figure are the correction we can make with the data in hand. We adopt the D/UD classifications whenever available (i.e., we exclude the Wide classifications for the duplicated objects), and apply the correction as shown by the green dots to objects classified only at the Wide depth to increase their |$P(\mathit {spiral})$| and |$P(\mathit {int.})$| to the D/UD depth. This leaves us 20686 unique objects.

It is interesting to note that the |$P(\mathit {spiral})$| distribution shows a clear concentration at |$P(\mathit {spiral})\sim 0$| and 1, and galaxies are relatively sparse at intermediate |$P(\mathit {spiral})$|⁠ . In contrast, the distribution of |$P(\mathit {int.})$| is more contiguous and there are many galaxies with intermediate |$P(\mathit {int.})$|⁠ , illustrating the difficulty of identifying interacting galaxies compared to the elliptical vs. spiral classifications.

3.2 Redshift dependence

Next, we look into the redshift dependence, as galaxies at higher redshifts suffer more significantly from cosmological dimming. We evaluate the dependence as a function of absolute magnitude; the reduction in the angular resolution on the physical scale at high redshift likely affects faint (and small) objects more severely than bright objects, and thus the effect is strongly dependent on magnitude as well. Figure  6 shows the fractions of spiral and interacting galaxies as a function of redshift and absolute magnitude in the r -band. We define spiral galaxies as those with |$P(\mathit {spiral})>0.79$| and interacting galaxies as those with |$P(\mathit {int.})>0.79$| (see the next subsection for the choice of the threshold).

Fraction of spiral galaxies (top) and interacting galaxies (bottom) plotted as a function of redshift and absolute magnitude. The left-hand panels are the original classification, and the right-hand ones are after applying the redshift corrections [equations ( 3 ) and ( 4 )]. The color bar in each panel shows the mapping between color and fraction. Only pixels with more than 10 objects are plotted. In each panel, the upper right-hand part is missing objects because both SDSS and GAMA are flux-limited surveys (i.e., intrinsically fainter galaxies at higher redshifts are not targeted). The missing bottom left-hand part is due to the small volume probed at low redshifts.

We first discuss the spiral fraction shown in the top panels. The spiral fraction is a function of magnitude in the sense that the spiral fraction decreases at brighter magnitudes. However, that is not our focus here as it is an intrinsic trend. We are interested in the redshift dependence. Over the narrow redshift range of 0 <  z  < 0.2, we can reasonably assume that the intrinsic spiral fraction does not significantly change (i.e., no significant evolution since z  = 0.2). Thus, any trend with redshift can be attributed to a classification bias. Willett et al. ( 2013 ) indeed observed strong redshift-dependence in Galaxy Zoo 2 (GZ2) in the sense that the spiral fraction decreases at higher redshifts and they made a statistical correction for it. Our spiral classification shows a similar but weaker trend with redshift. GALAXY CRUISE appears less biased than GZ2. This is likely because the HSC images are sufficiently deep to classify bright galaxies robustly across this redshift range (Tadaki et al. 2020 ). We apply only a weak correction to the spiral fraction:

where z is redshift. The maximum correction factor applied is only |$11\%$|⁠ , but it does reduce the redshift trend, as shown in the top right-hand panel of figure  6 . Note that, when |$P(\mathit {spiral})_{\rm corr}>1$|⁠ , we set |$P(\mathit {spiral})_{\rm corr}=1$|⁠ . For reference, the correction applied in GZ2 spans over a wide range, but it is typically |$\sim 50\%$|⁠ . There is still some redshift trend left in the top right-hand panel in the sense that the spiral fraction is very large at z ≲ 0.05. It might be due to the small volume probed. We find that there are few massive clusters at z  < 0.05, and galaxies at this redshift range are predominantly field galaxies ( ⁠|$\sim 1\%$| galaxies are in dense environments). On the other hand, about |$10\%$| of galaxies at 0.05 <  z  < 0.1 are in dense environments. We may be suffering from the morphology–density relation, which we will discuss in subsection  4.2 , in the top right-hand panel.

The bottom panels in figure  6 show the interaction fraction. There is a weak redshift trend even for very bright galaxies in the sense that the interaction fraction decreases toward higher redshifts. This is expected because tidal features are often diffuse and extended and we miss those features more easily at higher redshifts. We correct for the redshift dependence in the following way:

The correction is stronger than that for the spiral fraction and the maximum correction applied is |$25\%$|⁠ . As shown in the bottom right-hand panel, this correction significantly reduces the redshift trend for interacting galaxies. We note that we need both the redshift and depth corrections. The former reduces the redshift dependence of our classifications and the latter changes the overall normalization of |$P(\mathit {spiral})$| and |$P(\mathit {int.})$|⁠ .

We have confirmed that our main results in this paper are not affected significantly by whether or not we apply the corrections for the spiral and interaction fractions. We denote |$P(\mathit {spiral})_{\rm corr}$| and |$P(\mathit {int.})_{\rm corr}$| as simply |$P(\mathit {spiral})$| and |$P(\mathit {int.})$| respectively in what follows for the sake of simplicity.

3.3 Color–magnitude diagram

There may also be a bias in the participants’ classifications themselves as visual classifications are subjective. In order to reduce the subjectivity, it is a common practice to classify objects by multiple people independently (e.g., Fukugita et al. 2007 ). Although this is already done in GALAXY CRUISE as we merge classifications by many participants for each galaxy (see figure  3 ), it is still useful to check if there is a systematic bias especially because the classifications are made by non-professional astronomers. However, this is not a trivial question, because there is no truth-table of morphology for our sample and edge-cases are difficult even for professional astronomers.

In order to evaluate the classification accuracy roughly, we use colors as the truth table. That is, we assume that elliptical galaxies are on the red sequence and spiral galaxies are in the blue cloud. There are of course green-valley galaxies, which are often bulge-dominated spiral galaxies, and also spiral galaxies are not always blue; edge-on spirals are red, and there is a population of spiral galaxies with no sign of ongoing star formation (van den Bergh 1976 ; Couch et al. 1998 ; Masters et al. 2010 ; Shimakawa et al. 2022 ). However, overall there is a good correlation between morphology and color. For instance, Schawinski et al. ( 2009 ) showed that the vast majority ( ⁠|$96\%$|⁠ ) of the visually selected elliptical galaxies are red. Colors are a different attribute than morphology, but we use them as a proxy for morphology for now.

Figure  7 shows the rest-frame u − r color K -corrected to z  = 0.1 using the K -correction code by Blanton and Roweis ( 2007 ) plotted against absolute magnitude in the r band with galaxies color-coded according to their |$P(\mathit {spiral})$|⁠ . As can be seen, the red sequence and blue cloud are fairly well populated by elliptical and spiral galaxies, respectively. This demonstrates that the classifications are good at least to the first order. There are, however, galaxies with large |$P(\mathit {spiral})$| on the red sequence and those with low |$P(\mathit {spiral})$| in the blue cloud. Galaxies with intermediate |$P(\mathit {spiral})$| are located both on the red sequence and the blue cloud. They may not necessarily be wrong classifications as discussed above, but for now we assume that the colors are the truth table and make an attempt to quantify the classification accuracy below.

Rest-frame u − r color plotted against r -band absolute magnitude. Galaxies are color-coded according to their |$P(\mathit {spiral})$| as shown by the color bar. We split the red/blue galaxies at the dashed line.

The left-hand panel of figure  8 shows the |$P(\mathit {spiral})$| distributions of the red and blue galaxies. |$P(\mathit {spiral})$| of red galaxies is sharply peaked at |$P(\mathit {spiral})=0$| with a long and flat tail toward larger |$P(\mathit {spiral})$|⁠ . The distribution of blue galaxies is similar albeit with a broader wing around |$P(\mathit {spiral})=1$|⁠ . The tails indicate that there may be misclassifications. To characterize it better, we show in the right-hand panel the purity and completeness of spiral galaxies as a function of threshold |$P(\mathit {spiral})$|⁠ . Here we define the purity as

where N selected is the number of galaxies with |$P(\mathit {spiral})$| above a certain threshold. The threshold here is a free parameter and is taken in the horizontal axis. N blue, selected is the number of blue galaxies among the selected galaxies. Thus, the purity is the fraction of blue galaxies among galaxies selected with a threshold |$P(\mathit {spiral})$|⁠ . Likewise, we define completeness as

where N blue is the total number of blue galaxies and N blue,selected is the number of blue galaxies with |$P(\mathit {spiral})$| above the threshold.

Left: Distribution of |$P(\mathit {spiral})$| for red (open histogram) and blue (cross-hatched histogram) galaxies. Right: Purity and contamination rate plotted as a function of threshold |$P(\mathit {spiral})$|⁠ . For instance, the purity and contamination at |$P(\mathit {spiral})=0.2$| are for spiral galaxies defined as |$P(\mathit {spiral})>0.2$|⁠ . The vertical dashed line is the threshold adopted in this paper.

The purity shown in the right-hand panel increases with increasing the threshold |$P(\mathit {spiral})$|⁠ . This is expected because the contamination of elliptical galaxies decreases with more stringent |$P(\mathit {spiral})$| cuts. On the other hand, the completeness decreases with increasing threshold |$P(\mathit {spiral})$|⁠ . We want to be both pure and complete, but we need to make a compromise between the two in the presence of contaminating galaxies. Here we choose to take |$P(\mathit {spiral})=0.79$|⁠ , indicated by the vertical dashed line, as the threshold. This is the point where both purity and completeness are about |$75\%$|⁠ . We have confirmed that our primary conclusions in this paper remain unchanged if we perturb the threshold within a reasonable range, e.g., |$P(\mathit {spiral})>0.5$|⁠ .

A similar analysis will be useful for |$P(\mathit {int.})$|⁠ , but it is more difficult because there is no truth table or even a good proxy for interaction features. Rest-frame colors or other well-measured quantities do not work as a proxy. Furthermore, interaction features are intrinsically more difficult to identify than elliptical vs. spiral. Future missions with deeper and sharper imaging data over a wide area will allow us to characterize the classification accuracy for interacting galaxies better. For now, we choose to adopt the same threshold, |$P(\mathit {int.})>0.79$|⁠ , conservatively to define interacting galaxies. Once again, we have confirmed that main conclusions of the paper are not sensitive to the particular choice of the threshold within a reasonable range. We plan to carry out a campaign to classify galaxies from cosmological hydrodynamical simulations; we visualize these simulated galaxies accounting for various observational effects and insert them to the real HSC images, so that the participants can classify them (Bottrell et al. 2023 ). Because we know the merger histories of the simulated galaxies, we expect that we will be able to evaluate how well the participants can identify interacting galaxies as functions of, e.g., mass ratio and merger phase, and discover an optimal |$P(\mathit {int.})$| threshold.

An alternative approach to probability thresholding adopted here is simply to use the probabilities as weights; a galaxy with |$P(\mathit {spiral})=0.3$| is counted as 0.3 of a spiral galaxy. Which approach to adopt depends on science applications, and we briefly discuss this point in the Appendix.

3.4 Gallery of selected objects

To visualize the participants’ classifications, we present a gallery of elliptical and spiral galaxies in figure  9 . They are high-confidence elliptical and spiral galaxies and the classifications are indeed accurate. It will also be instructive to look at galaxies with intermediate |$P(\mathit {spiral})$|⁠ , shown in the bottom panel. Interestingly, these objects tend to be S0-like galaxies; we do not explicitly include S0s as a galaxy type in our classification scheme as they are difficult even for professional astronomers, but the participants’ classifications are actually useful to identify them and they are rightly classified as an intermediate type between elliptical and spiral. The color of these objects seems to be a mixture of red and blue and we find that they are indeed located on both the red sequence and the blue cloud.

Top: Gallery of galaxies with |$P(\mathit {spiral})<0.05$| (i.e., elliptical galaxies). Each image is 1′ on a side. Middle: Same as the top panel, but for spiral galaxies with |$P(\mathit {spiral})>0.95$|⁠ . Bottom: Galaxies with |$P(\mathit {spiral})\sim 0.5$|⁠ . These objects tend to be edge-on S0 galaxies.

Figure  10 shows galaxies with four distinct interaction features. These galaxies are first selected as |$P(\mathit {int.})>0.79$| and then 21 objects with the highest probabilities for each category are shown in these figures. It is impressive that the classifications are made very well; all these individual features are nicely captured by the participants. Galaxies with distorted shapes tend to have a nearby companion, which is likely interacting with the target galaxies. We find that some of the tidal tails seem to be distorted spiral arms. It is in some cases difficult to distinguish tidal tails from distorted arms, but this tendency of misclassifying distorted arms as tidal tails is a possible classification bias in GALAXY CRUISE. As for fans/shells, coherent caustic features are observed in all cases. Finally, ring galaxies are also all nice ring galaxies. Interestingly, they are all face-on ring galaxies; there should be edge-on (or polar) ring galaxies, but they are not identified. They might be confused with linear tails. We further discuss ring galaxies in subsection  4.5 .

Gallery of interacting galaxies. Shown are those with the highest probabilities for distorted shapes, tidal tails, fan/shell, and ring from top to bottom. Each panel is 1′ on a side.

Let us also comment on galaxies with lower |$P(\mathit {int.})$|⁠ . Visual inspections of a subset of |$P(\mathit {int.})\sim 0.5$| cases suggest that they are indeed a mixture of objects; some show a weak hint of distorted shapes, faint tails, etc, while others do not seem to exhibit clear interaction features. It seems wise to set the threshold to identify interacting galaxies well above 0.5 securely as we do in this paper. Another interesting class of objects is interacting galaxies with intermediate probabilities for individual interaction features. We recall that we allow the participants to choose multiple interaction features because some of the features can be difficult to distinguish and some galaxies actually exhibit multiple features. Visual inspections of interacting objects with the probability of each feature between 0.15 and 0.35 indeed show complex morphologies with multiple features. Tidal tails and shells are most frequently observed, but distorted shapes and ring-like features are also observed.

Finally, we show a collection of violent mergers in figure  11 . This is constructed from a joint selection of |$P(\mathit {not\_sure})>0.2$| from the first question and |$P(\mathit {int.})>0.79$| from the second question. As we discussed in subsection  2.4 , the “not sure” option was not used as we intended, but a certain fraction of the participants followed our intention and sorted objects with significantly disturbed morphology into the “not sure” category. The mergers in the figure are all in a violent merger phase and the original morphologies of the galaxies are indeed difficult to classify. In the analyses we present in section  4 , we primarily focus on interacting galaxies defined as |$P(\mathit {int.})>0.79$|⁠ , but we also include the violent merger subsample defined here. We note that the violent merger subsample comprises |$8\%$| of the interacting galaxy sample.

Galaxies with |$P(\mathit {int.})>0.79$| and |$P(\mathit {not\_sure})>0.2$|⁠ . Each panel is 1′ on a side.

3.5 Statistics of interacting galaxies

Before we discuss properties of the target galaxies in detail in the next section, we briefly summarize statistics of interacting galaxies in figure  12 . Overall, we find that about |$13\%$| of galaxies in our sample are interacting galaxies. The sample is not volume-limited and the number should not be over-interpreted, but the numbers here do not significantly change if we make a volume-limited sample with z  < 0.1 and M r  < −20.5. Also, the number is somewhat sensitive to the threshold |$P(\mathit {int.})$| value adopted (e.g., if we reduce it to 0.5, |$36\%$| of galaxies are interacting).

The leftmost bar shows the fraction of interacting galaxies among the entire sample. The number at the top of the bar and the one in the brackets are the fraction and its uncertainty. The bar is split into the upper and lower parts, which represent the elliptical and spiral fractions, respectively. The remaining bars are for the ring, fan, tail, and distortion features, and they are normalized such that the sum of their abundances equals to the abundance of interacting galaxies as shown in the leftmost bar. The meaning of the numbers and the upper/lower parts are the same as the leftmost one. For reference, the fractions of elliptical and spiral galaxies that are not interacting are 0.55 and 0.32, respectively (these numbers are normalized in the same way and hence the total fraction of non-interacting galaxies is 0.87).

It is interesting to note that the abundance of spiral galaxies relative to elliptical galaxies among the interacting galaxies is about 0.72, as shown in the leftmost bar, while this ratio is about 0.60 for non-interacting galaxies. The same trend holds for a volume-limited sample of M r  < −20.5 at z  < 0.1. Darg et al. ( 2010a ) found that the spiral-to-elliptical fraction among interacting galaxies is about twice as high as the global population. While direct comparisons cannot be made as the parent samples are constructed in different ways, we observe a similar (but weaker) trend.

Turning to individual features, we find that tidal tails and distortions are the most common interaction features and they comprise about 3 |$/$| 4 of all interacting galaxies. Spiral galaxies are more abundant than ellipticals for tidal tails, and it is likely due to the misclassifications of distorted spiral arms as tails. The remaining 1 |$/$| 4 of the interacting galaxies is split about equally into fan and ring features. Elliptical galaxies dominate over spiral galaxies here. We suspect that this is at least partly due to the general difficulty (not only for the participants but for professional astronomers as well) of identifying fan and ring features in the presence of spiral arms. These features are much easier to identify around elliptical galaxies. We are going to look at statistical properties of galaxies in GALAXY CRUISE in the next section and we keep these biases in mind.

Based on the catalog constructed in the previous section, we now move on to discuss statistical properties of the local galaxies. We first make a comparison with GZ2 (Willett et al. 2013 ) in subsection 4.1 . There is a more recent catalog based on deeper DECals data (Walmsley et al. 2022 ), but here we focus on GZ2 as it is the most widely used visual morphology catalog. 2 We then turn our attention to the environmental dependence of galaxy morphology in subsection 4.2 . In the following subsection 4.3 , we discuss AGNs and revisit the classic picture of AGN activities being triggered during galaxy interactions. Another activity that might be enhanced by mergers is star formation, and we look into it in subsection 4.4 . An interesting class of objects in GALAXY CRUISE is ring galaxies; the participants’ identification of ring galaxies is accurate, as shown in figure  10 , and we discuss their statistical properties in subsection 4.5 . Finally, based on a number of simple assumptions, we infer the merger rate from GALAXY CRUISE and discuss the mass growth rate of galaxies in subsection  4.6 .

4.1 Comparison with Galaxy Zoo 2

We cross-match our catalog with the GZ2 “Main” catalog (Willett et al. 2013 ) with positions. 8354 objects are matched within |${1{^{\prime \prime}_{.}}5}$|⁠ . 3 We first compare classifications from GZ2 with those from GALAXY CRUISE on an object-by-object basis.

The GZ2 classification scheme is different from ours, and there is no elliptical vs. spiral question in GZ2. However, the first question in GZ2, “Is the galaxy smooth and round with no sign of a disk?”, is sufficiently close and we use this question to compute |$P(\mathit {spiral})$| from GZ2. To be specific, a “No” to the question indicates that the galaxy is a spiral galaxy. There is a more specific question about the spiral feature later in the sequence of questions, “Is there any sign of a spiral arm pattern?”. Not all volunteers are asked this question. We could compute a conditional probability for spiral arm (Casteels et al. 2013 ), but we prefer to be simple here and use only the first question.

For interaction features, we use “Is there anything odd?” and the subsequent question about observable features to estimate |$P(\mathit {int.})$|⁠ . As some of the features are not a result of interaction (e.g., dust lane), we compute |$P(\mathit {int.})$| as a product of |$P(\mathit {there\ is\ something\ odd})$| and |$P(\mathit {ring}) + P(\mathit {disturbed}) + P(\mathit {irregular}) + P(\mathit {merger})$|⁠ .

We use GZ2’s classifications with weights to individual users (weighted fraction). There is a fraction corrected for the classification bias as a function of redshift (debiased fraction). However, as noted in Walmsley et al. ( 2022 ), these debiased fractions are useful only in a statistical sense and are not appropriate for object-by-object comparisons. For this reason, we focus on the weighted fraction here.

The left-hand panel in figure  13 compares |$P(\mathit {spiral})$|⁠ . There is a clump of objects at P ∼ 0 (bottom left-hand corner) and 1 (top right-hand corner). These are objects with obvious morphology and the classifications agree well. However, there is a significant population of galaxies in the bottom-right part of the figure; GALAXY CRUISE classifies them as spiral galaxies, while GZ2 classifies them as elliptical. We randomly draw objects from the bottom right-hand corner and show their postage stamps in figure  14 . All of these galaxies are clearly spiral galaxies. The reason for the discrepant classifications is simply the image quality; the SDSS images do not have sufficient depth and resolution to identify the spiral feature.

Comparisons between GZ2 and GALAXY CRUISE. The left-hand figure shows |$P(\mathit {spiral})$| from GZ2 plotted against that from GALAXY CRUISE. The dots are the weighted fraction from GZ2. The dashed line shows the one-to-one correspondence. The right-hand figure is for |$P(\mathit {int.})$| and the meanings of the symbols are the same as in the left-hand figure.

Randomly drawn objects with P GC ( spiral ) ∼ 1 and P GZ 2 ( spiral ) ∼ 0. The top row is the SDSS images used for GZ2 and the bottom row is GALAXY CRUISE for the same objects. Each image is 48″ on a side.

The right-hand panel in figure  13 shows that there is also a significant scatter in the identifications of interacting galaxies. The top left-hand corner is empty and there is a large population again in the bottom right-hand corner. Figure  15 shows postage stamps of randomly drawn objects from the bottom right-hand corner, and all these galaxies exhibit a clear feature to indicate interactions. The image depth is again the main reason for the discrepancy. These figures demonstrate that the classifications from GALAXY CRUISE are more accurate than those from GZ2.

Same as in figure  14 but for interacting galaxies.

Finally, we look at the fraction of spiral galaxies and interacting galaxies as a function of absolute magnitude in figure  16 . We recall that we define spiral galaxies as galaxies with |$P(\mathit {spiral})>0.79$|⁠ . We adopt the same definition also for GZ2 to be fair. The left-hand panel shows the well-known trend that the intrinsically bright (∼massive) galaxies tend to be early-type galaxies, while faint galaxies are more populated by late-type galaxies. The overall normalization of the spiral fraction is different between GALAXY CRUISE and GZ2 in the sense that the spiral galaxies are more abundant in GALAXY CRUISE. This trend holds even when we use the debiased fraction from GZ2 (we use the debiased fraction here because this is a statistical comparison). We note that, if we define spiral galaxies as those with |$P(\mathit {spiral})>0.5$|⁠ , GALAXY CRUISE and GZ2 debiased fractions are in good agreement, except for the magnitude range at M r ≳ −21, where GZ2 falls below GALAXY CRUISE. Baldry et al. ( 2004 ) examined the relative abundance of red and blue galaxies in the local Universe and showed that the fraction of red galaxies rises slowly from M r  = −18 to −22, and then sharply rises to almost unity at the brightest magnitudes. While we plot the spiral fraction (∼ fraction of blue galaxies) in figure  16 , the trend in the figure is fully consistent with the finding by Baldry et al. ( 2004 ). Colors are a different property from morphology as noted earlier, but the agreement here is reassuring because this strongly suggests that the participants’ classifications are accurate.

Left: Fraction of spiral galaxies plotted against r -band absolute magnitude. The filled circles are GALAXY CRUISE (GC), and the open circles are GZ2. For GZ2, we show the spiral fraction using the weighed fraction (darker symbol) and debiased fraction (lighter symbol). The error bars show the Poisson uncertainty. Right: Fraction of interacting galaxies as a function of the r -band absolute magnitude. The meaning of the symbols are the same as in the left-hand panel.

The right-hand panel shows the fraction of interacting galaxies. We recall again that the interacting galaxies are conservatively defined as those with |$P(\mathit {int.})>0.79$| and the same definition is used for GZ2. The same trend as in the left-hand panel can be seen; the interaction fraction is higher in GALAXY CRUISE. Interestingly, the weighted fraction and debiased fraction both yield similarly low fractions. The fractions remain low even if we define interacting galaxies as |$P(\mathit {int.})>0.5$|⁠ .

GALAXY CRUISE’s interaction fraction seems to show a declining trend with magnitudes; fainter galaxies are less likely to exhibit interaction features. This luminosity dependence is consistent with previous work (e.g., Patton & Atfield 2008 ). It is also consistent with Rodriguez-Gomez et al. ( 2015 ), who examined the major merger rate in the Illustris Simulation (Genel et al. 2014 ; Vogelsberger et al. 2014 ) and showed that the rate is a strong function of stellar mass in the sense that more massive galaxies experience more mergers. The mass dependence of merger rate indicates that more massive galaxies have a larger fraction of accreted (ex-situ) stars (Rodriguez-Gomez et al. 2016 ). Our result here points to the same picture.

As described in subsection 2.3 , we mistakenly used HSC photometry to select targets. A preliminary analysis of the data from the second season shows that we tend to miss spiral galaxies and interacting galaxies from the sample because they have significant structure. The spiral and interaction fractions discussed here should thus be interpreted as a lower limit. The ongoing second season of GALAXY CRUISE will get rid of this problem and give us better estimates of these fractions.

4.2 Dependence on environment

Another interesting question about our targets is their environmental dependence. We characterize their local environment and discuss how the spiral fraction and interaction fraction change with environment. The former is the well-known morphology–density relation (Dressler 1980 ) and we first check if the participants’ classification can reproduce it.

To compute the local environment, we use spectroscopic redshifts from SDSS and construct a volume-limited sample at z  < 0.1 with M r  < −20.5. We then estimate a distance to the fifth-nearest neighbor within a radial velocity slice of ±1000 km s −1 from each of our targets, and translate it into local density as |$5/(\pi r_5^2)$|⁠ , where r 5 is the projected physical distance to the fifth-nearest neighbor. We apply the same redshift and magnitude cuts to our targets.

The left-hand panel of figure 17 shows the fraction of spiral galaxies as a function of local density. We observe the spiral fraction decreasing with increasing local density. In the densest region, the spiral fraction is below |$20\%$|⁠ . We observe a hint of a break in the relation at density ∼2 Mpc −2 , above which the fraction decreases more rapidly. The break density corresponds to the virial radius of groups and clusters (Tanaka et al. 2004 ). The participants’ classifications nicely reproduce the morphology–density relation.

Left: Morphology–density relation. The fraction of spiral galaxies is plotted against local density. Only bins with more than five objects are plotted. Right: Fraction of interacting galaxies as a function of local density. The lighter symbols are for the violent merger subsample.

The right-hand panel of figure  17 shows the fraction of interacting galaxies as a function of local density. We observe a decreasing trend with increasing local density as in the spiral fraction, although the scatter is larger. The trend remains the same with different bin sizes. The classical idea about interactions is that group environments are efficient places for interactions because relative velocities between galaxies is not very high, while more massive clusters are not very efficient due to their high velocities. This idea is supported by numerical simulations (Jian et al. 2012 ). The local density alone cannot distinguish groups from clusters (Tanaka et al. 2004 ) and we have a mix of groups and clusters in figure  17 . Our result here implies that interactions occur, on average, less frequently in high-density regions.

This possible decreasing trend is not consistent with some of the previous studies (e.g., Lin et al. 2010 ; de Ravel et al. 2011 ), which reported an increasing fraction of interacting galaxies with increasing local density. One might suspect that the difference is potentially due to the different environments probed. Some of the previous work is based on a relatively small data set such as zCOSMOS (Lilly et al. 2007 ) and DEEP2 (Newman et al. 2013 ), and thus their interacting galaxies are mostly located in groups rather than in clusters. On the other hand, we use a larger data set and we have a larger fraction of cluster galaxies. As a quick check, we perform the same analysis as in figure  17 with density estimated with the 20th nearest neighbor so that we are more sensitive to the large-scale environment. We find that the same trend holds; the interaction fraction decreases in high-density environments. This suggests that the dominance of groups vs. clusters is not the main driver of the observed trend. We also examine the distribution of the violent mergers (subsection  3.4 ) but they do not seem to prefer any specific environment.

The difference is at least in part due to different methods employed to identify interacting galaxies. Many of the previous studies are based on galaxy pairs, which probe the pre-merger phase, while our work is based on visual signatures of interactions, which are also sensitive to later phases of interactions. Also, it is possible that the timescale over which tidal features are observable depends on environment. For instance, many galaxies are traveling fast in a small volume in clusters, tidal features may be short-lived compared to those around isolated galaxies. We speculate that the combination of these reasons may be able to explain the observed difference. The error bars in figure  17 are still large and an increased sample of interacting galaxies from, e.g., machine-learning based on the GALAXY CRUISE classifications will allow us to explore the origin of the reduced frequency of interactions in dense regions further.

4.3 AGN fraction

Next, we look into the long-standing question of whether galaxy–galaxy interactions trigger AGN activities (Keel et al. 1985 ). AGNs can be identified by a variety of methods, e.g., X-rays, IR colors, variability, etc. As all of our targets have spectra from SDSS and GAMA, we use emission line intensity ratios suggested by Baldwin, Phillips, and Terlevich ( 1981 ). In order to separate AGNs from star-forming galaxies, we use the threshold proposed by Kauffmann et al. ( 2003 ). This diagnostics involve four emission lines: Hβ, [O  iii ] (λ5007), [N  ii ] (λ6583), and Hα. We require that all of these four lines are measured at >3σ. Among the 20686 galaxies that GALAXY CRUISE targeted, 7994 galaxies passed the condition. The other galaxies are considered normal (i.e., non-AGN host) galaxies, although some of them may harbor obscured or very weak AGNs.

To get a rough picture of the AGN population in our sample, we first show in figure  18 the fraction of AGNs as a function of magnitude. The exact AGN fractions here should be taken with caution because they are not from a volume-limited sample. Nonetheless, relative differences between the interacting and non-interacting galaxies are interesting; the AGN fraction is consistently higher for interacting galaxies than for non-interacting galaxies at relatively bright magnitudes ( M r ≲ −21), while the fraction is consistent at fainter magnitudes. This trend is not sensitive to a particular choice of the bin size. The observed higher AGN fraction among interacting galaxies is in qualitative agreement with Ellison et al. ( 2013 ), who observed that AGNs are more common in galaxy pairs with smaller angular separations. We see a hint of the very high AGN fraction among bright violent mergers, which may also be in line with Ellison et al. ( 2013 ) as the closest pairs are more likely to be strongly disturbed. Goulding et al. ( 2018 ) addressed the AGN enhancement using the HSC-SSP imaging data and found that AGNs are a factor of 2–8 times more abundant. While the differences in the way that mergers and AGNs are identified again make it difficult to make quantitative comparisons, we are in qualitative agreement.

AGN fraction plotted against r -band absolute magnitude. The filled and open circles represent interacting and non-interacting galaxies, respectively. The lighter points are the violent merger subsample. The error bars show the Poisson errors. Only bins with more than five objects are plotted here.

We then make an attempt to see whether any one of the specific interaction features is more important for triggering AGN activities. Figure  19 shows AGN fractions for each interaction feature for a volume-limited sample with z  < 0.1 and M r  < −20.5. We find that the AGN fraction is clearly higher among interacting galaxies; |$35\%$| of interacting galaxies exhibit a sign of AGN, while the fraction is |$28\%$| for non-interacting galaxies. This suggests that interactions can indeed trigger AGN activities at a certain rate. If we turn our attention to individual interaction features, we find that the relative fractions of fan, tail, and distortion features to the total AGN fraction are similar to the overall feature fractions shown in figure  12 ; tail and distortions are about the same fraction and their sum comprises about 3 |$/$| 4 of the interacting galaxies, and the ring and fan features are about |$1/3$| of each of these features. A possible interpretation is that the orbital parameters of infalling galaxies are not important for triggering AGN activities. It may simply be the tidal disturbance that induces AGN activities. We note that our interaction galaxies include many phases of mergers and interactions; some are in pre-merger phases, while others are post-mergers. There may be the phase dependence of AGN activities; in fact, the violent mergers (subsection  3.4 ) seem to show a higher AGN fraction of 0.42 ± 0.07, indicating that the strong tidal field may be the key to triggering AGNs. The uncertainty is large at this point, and more detailed investigations of merger phases may shed further light on the physical process to activate AGNs.

Fraction of AGNs for interacting and non-interacting galaxies (leftmost two bars). The overall AGN fractions are higher than those shown in figure  18 , but here we use a volume-limited sample at z  < 0.1. Interacting galaxies are split into different features as shown in the remaining four bars to the right. These feature fractions are normalized such that the sum of each feature equals the AGN fraction (i.e., the sum is 0.347). The rightmost bar is the violent merger subsample. The numbers on the top of each bar are the fraction and its statistical uncertainty, respectively.

Our analysis makes it clear that galaxy–galaxy interactions can trigger AGN activities, but galaxies without any clear sign of interactions also exhibit AGN activities and their fraction is as high as |$28\%$|⁠ . This suggests that interactions are not the only mechanisms to trigger AGNs, and secular processes are one of the primary ways to activate AGNs. Ellison et al. ( 2019 ) reported that infrared-selected AGNs show a sign of disturbance more frequently than optically selected AGNs. It would be interesting to look at AGNs selected in various ways and investigate this issue further. We leave it for future work.

4.4 SFR enhancement during interaction

Another activity that may be enhanced during interactions is star formation (e.g., Nikolic et al. 2004 ; Ellison et al. 2008 , 2013 ) and we focus on star formation activities of interacting vs. non-interacting galaxies in this subsection. There are multiple ways to infer star formation rates (SFRs) of galaxies, but here we adopt the one based on the Hα luminosity. We translate the Hα luminosity into SFR using the relationship from Murphy et al. ( 2011 ). We correct for the dust attenuation when Hβ is measured at >3σ using the Cardelli et al. ( 1989 ) dust attenuation curve. No correction is made for objects with weaker Hβ. We apply no correction for the fiber aperture loss as we are interested in relative differences between interacting and non-interacting galaxies.

SFR is known to correlate significantly with stellar mass (Brinchmann et al. 2004 ). As the interaction fraction is a function of absolute magnitude (and stellar mass as well) as shown in figure  16 , a direct comparison of SFRs is not easy to interpret. We utilize the fact that the SFR– M * correlation is roughly linear (Speagle et al. 2014 ) and we compare the distributions of specific SFR (sSFR) defined as SFR |$/$| M * . The relation is not completely linear, but we have confirmed that the small non-linearity does not alter our conclusion here. We use stellar mass estimates from Ahn et al. ( 2014 ) (GranadaWideDust) for SDSS galaxies. We exclude the GAMA galaxies to avoid potential systematic effects arising from different stellar mass estimates, but the fraction of GAMA galaxies is only ∼10% of the entire sample (subsection  2.3 ) and whether we include them or not does not change our results.

Figure  20 shows the sSFR distributions of the two populations. For the non-interacting galaxies, we construct a mass-matched sample; we randomly draw a subset of non-interacting galaxies so that their stellar mass distribution is consistent with that of interacting galaxies. The plot is for the entire sample, but the plot remains similar if we use a volume-limited sample with z  < 0.1 and M * > 10 10.5   M ⊙ . Note that AGNs identified in the previous subsection are excluded from the figure so that we do not misinterpret AGN emission as star formation activity. Note as well that we focus on spiral galaxies with |$P(\mathit {spiral})>0.79$| because we are interested in the enhancement of star formation activities. We confirm that elliptical galaxies in our sample do not exhibit a significant sSFR enhancement.

Specific SFR distributions of interacting (filled points) and non-interacting (open points) spiral galaxies. The violent merger subsample is shown in the lighter color. The error bars show the Poisson error. The arrows indicate the median of the distribution (−11.19, −10.88, and −10.39 for non-interacting, interacting, and violent mergers, respectively).

The sSFR distributions are clearly different; interacting galaxies show an sSFR enhancement by about a factor of 2. The overall shapes of the distributions are similar, but there is a clear offset between them (the median offset is 0.31 dex). This enhancement is consistent with the finding by Darg et al. ( 2010b ), Ellison et al. ( 2013 ), and Bickley et al. ( 2022 ), among others, although their interacting galaxies are selected in different ways. The enhancement is also consistent with recent hydrodynamical simulations (Hani et al. 2020 ). As we have classifications of various tidal features, we can address which feature is more important. It seems that the offset is primarily driven by galaxies with tidal tails. Galaxies with distorted shapes also contribute to the overall trend. The statistics are too poor for galaxies with ring and fan features to draw any conclusions about them. As noted earlier, machine-learning techniques based on the current sample will be able to make a larger sample, which will allow us to characterize the relative contribution of each feature more securely. However, it is fairly robust already from the current sample that sSFR is boosted by a factor of ∼2 by interactions. Interestingly, the violent mergers (subsection  3.4 ) show an even larger boost of a factor of ∼6 (0.8 dex), indicating that both star formation and AGN activities (see the previous section) are most significantly enhanced during the violent merger phase.

4.5 Ring galaxies

Few and Madore ( 1986 ) introduced two classes of ring galaxies: O-type, which is a smooth and regular ring with the galaxy nucleus at its center, and P-type, which is often a knotted, asymmetric ring and the nucleus is not necessarily at the center. It is interesting that the ring galaxies shown in figure  10 are mostly O-type rings. Some of them might be pseudo-rings (Shimakawa et al. 2022 ) and there is certainly a bias toward face-on rings in our classifications because there is no, e.g., polar-ring galaxy. This is at least in part due to the limited resolution of our imaging data on the physical scale (our sample extends out to z  = 0.2). Buta ( 2017 ) discussed biases in ring galaxies selected from GZ2 and made a similar finding; most ring galaxies are those with bright, large outer rings with a nearly face-on configuration. While our resolution is better than SDSS on which GZ2 is based, our sample likely suffers from the same bias. However, the subdominance of P-type rings is still interesting; if there were P-type rings of similar angular sizes, they would have been identified. Of course, we cannot deny the possibility that the subdominance of P-type rings is due to classification bias, in that it is easier for the participants to identify smooth and symmetric O-type rings. This caveat should be kept in mind.

Elagali et al. ( 2018 ) examined ring galaxies from the EAGLE simulation (Schaye et al. 2015 ). They found that about |$80\%$| of the ring galaxies originate from interactions. Interestingly, their ring galaxies seem to be mostly P-type rings. This may be expected as collisional rings may show star formation activities induced by the collision. The dominance of O-type rings in our sample is in contrast to their result. We visually inspect galaxies with |$P(\mathit {int.})>0.79$| and P ( ring ) > 0.5 and find that P-type rings comprise only about |$10\%$|⁠ . We further find that more than |$80\%$| of the ring galaxies are classified as red galaxies on the basis of the color–magnitude diagram, and the remaining galaxies tend to be located in the green valley. This indicates that most ring galaxies are not actively forming stars. O-type rings can be formed by resonances due to the central bar, and the observed red color may be consistent with the secular formation mechanism. However, many of the ring galaxies in our sample do not seem to exhibit a clear bar structure. Elagali et al. ( 2018 ) found that about |$20\%$| of the ring galaxies hold the ring structure for as long as 2 Gyr or more. As our imaging data are fairly deep, we may be preferentially detecting these long-lived rings, in which star formation activities have already ceased.

In any case, more detailed investigations are clearly needed to go beyond these speculations. For instance, close comparisons with recent hydrodynamical simulation will be useful. Also, machine-learning techniques make it possible to identify galaxies in simulations that look similar to a given observed galaxy. By identifying counterparts of the ring galaxies in simulations and going back in time, we will be able to get a better handle on the origin of these galaxies.

4.6 Merger rate

Finally, we discuss the merger rate. It is not trivial to estimate the merger rate from our data due to a number of observational complexities; our interacting galaxies include both pre- and post-mergers, some of them might be undergoing fly-by interaction and will not merge, etc. Here, we make simplified assumptions and attempt to make a rough estimate of the merger rate.

We infer the merge rate as

where f int. is the observed interaction fraction and T is the typical timescale over which the interaction features can be observed with our data. Some of the merger features we have identified may be due to multiple merger events, but we assume that they are due to a single event for simplicity.

It is difficult to make a robust estimate of T as the survival time of interaction features depend, for instance, on orbital parameters of infalling galaxies. A radial tail is likely short-lived because it can be destroyed as it passes through the strong tidal field around the potential center, while tangential tails (arcs) may live longer. There may also be environmental dependence, as we discussed earlier. We assume that interaction features are observable for the order of the dynamical timescale, T ∼ 1 Gyr. Equation ( 7 ) is then

The inferred merger rate is shown in figure  21 for a volume-limited sample with z  < 0.1 and M * > 10 10.5   M ⊙ . For comparison, we show predictions from the Illustris simulation (Rodriguez-Gomez et al. 2015 ). The merger rate is a strong function of mass ratio between merging galaxies in the sense that minor mergers are more common than major mergers. Here, we find that our merger rate corresponds approximately to μ > 0.04 (i.e., mass ratios larger than ∼1 |$/$| 25). This large mass ratio may not be too surprising because our imaging is very deep and is able to detect faint features due to minor mergers. Note that the overall normalization of our merger rate is sensitive to the assumptions employed and it should be regarded only as a rough estimate.

Merger rate plotted against stellar mass. The points show the observed merger rate inferred from equation ( 8 ). The associated error bars include only the Poisson error. The dotted curves are from Rodriguez-Gomez et al. ( 2015 ) based on the Illustris simulation. The different curves mean different range of mass ratios (μ) between the merging galaxies considered. For instance, the curve for μ > 0.1 is for all mergers with mass ratio of 0.1 or larger.

Based on the merger rate, we can infer the mass growth rate as

where M 1 is the mass of the central galaxy and 〈 M 2 〉 is the typical mass of an accreting galaxy. For f int.  = 0.13 (which is the interaction fraction at |$M_*=10^{11}\ M_{\odot}$|⁠ ) and 〈 M 2 〉 |$/$| M 1  = 0.04, the (fractional) growth rate inferred here is dM 1 |$/$| M 1 ∼ 0.005 Gyr −1 , which is smaller than that estimated by Tal et al. ( 2009 ) by more than an order of magnitude. One of the main differences is in f int. ; Tal et al. ( 2009 ) find that |$70\%$| of galaxies they observed show a sign of interaction, while we find |$13\%$|⁠ . As noted earlier, the overall normalization has a significant uncertainty both in our work and also in Tal et al. ( 2009 ), and we do not peruse the difference further at this point.

Assuming the redshift evolution of the merger rate from Rodriguez-Gomez et al. ( 2015 ) and considering all mergers with a mass ratio larger than 0.04, we expect a mass growth of a factor of ∼1.4, since z  = 1 for galaxies with M *  = 10 11   M ⊙ at z  = 0. This is in broad agreement with Davidzon et al. ( 2017 ), who evaluated the evolution of the stellar mass function of galaxies and found the observed growth of the characteristic stellar mass since z ∼ 1 is ∼0.2 dex.

Despite the simplified assumptions employed, the inferred mass growth rate roughly reproduces the observed mass growth. Our approach here is complementary to many of the literature results based on galaxy pairs (e.g., Bundy et al. 2009 ; López-Sanjuan et al. 2013 ) due to different systematic effects. Our merger rates are, however, sensitive to the assumptions employed as well as the threshold adopted to define interacting galaxies, |$P(\mathit {int.})$|⁠ . As we discussed in subsection  3.3 , we do not know the best |$P(\mathit {int.})$| to cut at. The adopted threshold is rather conservative and a more optimal threshold may change the overall normalization.

We have presented our community science project GALAXY CRUISE and its first science results. We focus on morphologies of nearby galaxies with spectroscopic redshifts from SDSS and GAMA with an emphasis on interaction features. The combination of the depth, seeing, and wide area coverage of the HSC-SSP data surpasses the previous community science projects on similar subjects; we are able to identify spiral features and interaction features that were missed in previous projects. After careful screening and statistical corrections to the classifications, the catalog is ready for various scientific explorations.

We have first compared with previous results from GZ2. We confirm that we make more accurate morphological classifications of galaxies than the previous work thanks to the better imaging data. We successfully reproduce the morphology–density relation. We find that tidal tails and distortion are the most frequently observed interaction features and they comprise about 3 |$/$| 4 of all the interaction features. The remainder is equally shared between the ring and fan features. The interacting galaxies seem to decrease in high density regions, which is not necessarily consistent with previous findings. We suspect that the different methods used to identify interacting galaxies can at least partly explain the difference.

As all of our targets have spectra, we apply the BPT diagnostics to identify AGNs. We demonstrate that interactions can trigger AGN activities; the fraction of AGN host galaxies among interacting galaxies is clearly higher than that of non-interacting galaxies. However, about a quarter of non-interacting galaxies harbor AGNs, which indicate that interactions are not the only mechanism to trigger AGNs. We also demonstrate that interactions enhance star formation activities of spiral galaxies by about a factor of 2. Both AGN and star formation activities are even more enhanced among violent mergers, indicating that the strong tidal field is driving this enhancement.

Finally, we have made an attempt to infer the merger rate. We find that our merger rate is similar to that inferred from the Illustris simulation for mass ratios greater than 1 : 25. The inferred mass growth rate due to mergers since z  = 1 is in good agreement with the observed evolution of the stellar mass function since z  = 1. As many of the previous studies inferred the merger rate from pair statistics, our analysis in this paper is highly complementary because interacting galaxies are identified in a different way.

This work is based on classifications collected during the first season of GALAXY CRUISE. We make the merged catalog used in this paper publicly available at the GALAXY CRUISE website, so that the world-wide community benefits from the large number of high-quality classifications. As emphasized throughout the paper, the HSC-SSP images are much deeper than those adopted in previous projects on similar subjects. Our catalog here can be used as a training data set to develop machine-learning algorithms and apply them to galaxies that are not targeted in this paper (e.g., those without spectroscopic redshifts and those covered in releases after PDR2), which will significantly increase the sample of interacting galaxies. This is just one example of how to use the catalog, and it can be used for a wide variety of purposes.

Currently, the second season of GALAXY CRUISE is underway. Given the good classification accuracy achieved in the first season, we choose to include fainter galaxies. We also eliminate the photometry problem that was erroneously introduced. The second season data will thus extend the magnitude range and help us unveil the nature of fainter galaxies in the local Universe. We will report on updated analyses using the second season data in our future paper, and the classifications will be released to the community in due course as well.

This paper is based on morphological classifications of galaxies by the GALAXY CRUISE volunteers, without whom the work would not have been possible. We deeply appreciate their efforts and contributions. We thank Miraikan, the National Museum of Emerging Science and Innovation, for their support and assistance during the public experiments. The experiments were essential for us to define the classification scheme and make a tutorial course. This work is supported by JSPS KAKENHI Grant Numbers JP 22H01270 and JSPS 22K14078. We thank the anonymous referee for constructive comments, which helped improve the paper.

The Hyper Suprime-Cam (HSC) collaboration includes the astronomical communities of Japan and Taiwan, and Princeton University. The HSC instrumentation and software were developed by the National Astronomical Observatory of Japan (NAOJ), the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), the University of Tokyo, the High Energy Accelerator Research Organization (KEK), the Academia Sinica Institute for Astronomy and Astrophysics in Taiwan (ASIAA), and Princeton University. Funding was contributed by the FIRST program from Japanese Cabinet Office, the Ministry of Education, Culture, Sports, Science and Technology (MEXT), the Japan Society for the Promotion of Science (JSPS), Japan Science and Technology Agency (JST), the Toray Science Foundation, NAOJ, Kavli IPMU, KEK, ASIAA, and Princeton University.

This paper is based on data collected at the Subaru Telescope and retrieved from the HSC data archive system, which is operated by Subaru Telescope and Astronomy Data Center at NAOJ. Data analysis was in part carried out with the cooperation of Center for Computational Astrophysics at NAOJ. We are honored and grateful for the opportunity of observing the Universe from Maunakea, which has the cultural, historical and natural significance in Hawaii.

This paper makes use of software developed for Vera C. Rubin Observatory. We thank the Rubin Observatory for making their code available as free software at 〈 http://pipelines.lsst.io/ 〉.

The Pan-STARRS1 Surveys (PS1) and the PS1 public science archive have been made possible through contributions by the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project Office, the Max-Planck Society and its participating institutes, the Max Planck Institute for Astronomy, Heidelberg and the Max Planck Institute for Extraterrestrial Physics, Garching, The Johns Hopkins University, Durham University, the University of Edinburgh, the Queen’s University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Incorporated, the National Central University of Taiwan, the Space Telescope Science Institute, the National Aeronautics and Space Administration under Grant No. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate, the National Science Foundation Grant No. AST-1238877, the University of Maryland, Eotvos Lorand University (ELTE), the Los Alamos National Laboratory, and the Gordon and Betty Moore Foundation.

Funding for the Sloan Digital Sky Survey IV has been provided by the Alfred P. Sloan Foundation, the U.S. Department of Energy Office of Science, and the Participating Institutions.

SDSS-IV acknowledges support and resources from the Center for High Performance Computing at the University of Utah. The SDSS website is 〈 www.sdss4.org 〉.

SDSS-IV is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS Collaboration including the Brazilian Participation Group, the Carnegie Institution for Science, Carnegie Mellon University, Center for Astrophysics/Harvard & Smithsonian, the Chilean Participation Group, the French Participation Group, Instituto de Astrofísica de Canarias, The Johns Hopkins University, Kavli Institute for the Physics and Mathematics of the Universe (IPMU) / University of Tokyo, the Korean Participation Group, Lawrence Berkeley National Laboratory, Leibniz Institut für Astrophysik Potsdam (AIP), Max-Planck-Institut für Astronomie (MPIA Heidelberg), Max-Planck-Institut für Astrophysik (MPA Garching), Max-Planck-Institut für Extraterrestrische Physik (MPE), National Astronomical Observatories of China, New Mexico State University, New York University, University of Notre Dame, Observatário Nacional / MCTI, The Ohio State University, Pennsylvania State University, Shanghai Astronomical Observatory, United Kingdom Participation Group, Universidad Nacional Autónoma de México, University of Arizona, University of Colorado Boulder, University of Oxford, University of Portsmouth, University of Utah, University of Virginia, University of Washington, University of Wisconsin, Vanderbilt University, and Yale University.

GAMA is a joint European-Australasian project based around a spectroscopic campaign using the Anglo-Australian Telescope. The GAMA input catalogue is based on data taken from the Sloan Digital Sky Survey and the UKIRT Infrared Deep Sky Survey. Complementary imaging of the GAMA regions is being obtained by a number of independent survey programmes including GALEX MIS, VST KiDS, VISTA VIKING, WISE, Herschel-ATLAS, GMRT and ASKAP providing UV to radio coverage. GAMA is funded by the STFC (UK), the ARC (Australia), the AAO, and the participating institutions. The GAMA website is 〈 http://www.gama-survey.org/ 〉.

Appendix. Using probabilities as weights

In the main body of the paper, we define galaxies with |$P(\mathit {spiral})>0.79$| as spiral galaxies. Instead of thresholding the probability, one can consider simply using the probability as a weight. For instance, a galaxy with |$P(\mathit {spiral})=0.3$| can be counted as 0.3 spiral galaxy. Which approach to adopt depends on what one wishes to look at, but we here briefly discuss how the spiral fraction changes between the two approaches. Figure  22 shows the spiral fraction as a function of M r using |$P(\mathit {spiral})$| as a weight. This figure can be compared with figure  16 , which is based on thresholding. Overall, the spiral fraction from GALAXY CRUISE does not change significantly. This is not surprising because |$P(\mathit {spiral})$| is sharply peaked at 0 and 1 as shown in figure  8 and thresholding at |$P(\mathit {spiral})=0.79$| simply splits these peaks into two. On the other hand, |$P(\mathit {spiral})$| from GZ2 is more contiguous, and both weighted and debiased fractions increase when |$P(\mathit {spiral})$| is used as a weight. The debiased fractions are in better agreement with GALAXY CRUISE, but there is still the under-abundance of faint spirals.

Same as figure  16 , but here |$P(\mathit {spiral})$| is used as a weight instead of thresholding |$P(\mathit {spiral})$| to define spiral galaxies.

This paper is made possible by about 10000 participants and they are individually acknowledged at 〈 https://galaxycruise.mtk.nao.ac.jp/en/commendations.html 〉.

We launched the Japanese site on 2019 November 1 and the English site on 2020 February 19. The first season ran through 2022 April 25, and the second season is in progress at the time of writing. The GALAXY CRUISE website is 〈 https://galaxycruise.mtk.nao.ac.jp/ 〉.

Our results below remain essentially the same if we compare against DECals because HSC-SSP is deeper than DECals by more than 2 mag.

More than half of our targets do not match with GZ2, although both samples are based on the spectroscopic redshifts from SDSS. This is primarily due to GZ2’s cut on the half-light Petrosian magnitude being brighter than 17.0 in the r -band (Willett et al. 2013 ). Our sample includes fainter galaxies.

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Astrophysics > Astrophysics of Galaxies

Title: galaxy cruise: deep insights into interacting galaxies in the local universe.

Abstract: We present the first results from GALAXY CRUISE, a community (or citizen) science project based on data from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). The current paradigm of galaxy evolution suggests that galaxies grow hierarchically via mergers, but our observational understanding of the role of mergers is still limited. The data from HSC-SSP are ideally suited to improve our understanding with improved identifications of interacting galaxies thanks to the superb depth and image quality of HSC-SSP. We have launched a community science project, GALAXY CRUISE, in 2019 and collected over 2 million independent classifications of 20,686 galaxies at z < 0.2. We first characterize the accuracy of the participants' classifications and demonstrate that it surpasses previous studies based on shallower imaging data. We then investigate various aspects of interacting galaxies in detail. We show that there is a clear sign of enhanced activities of super massive black holes and star formation in interacting galaxies compared to those in isolated galaxies. The enhancement seems particularly strong for galaxies undergoing violent merger. We also show that the mass growth rate inferred from our results is roughly consistent with the observed evolution of the stellar mass function. The 2nd season of GALAXY CRUISE is currently under way and we conclude with future prospects. We make the morphological classification catalog used in this paper publicly available at the GALAXY CRUISE website, which will be particularly useful for machine-learning applications.

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Galaxy Cruise: Trial by Leisure (Galaxy Cruise - Complete Series) Hardcover – April 15, 2022

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Caught in the ultimate tourist trap... Exiled in a bizarre galaxy, Captain Leo MacGavin is desperate to find the mythical alien technology that will send his crew home. But when his crush on the ship’s hologram causes him to follow a shady lead, the Americano Grande is trapped by a pair of all-powerful space gods. Duty-bound to protect their planet, these sexy centurions refuse to allow passage until the crew proves they are a harmless pleasure vessel. Fortunately, all they have to do is show their passengers a good time. Unfortunately, their passengers are a delegation of pregnant aliens with a conceited leader, xenophobic attitudes, and rapidly approaching due dates. When one of the dreamy deities puts the moves on Leo’s love interest, can he set his feelings aside to save the ship? Can the crew satisfy the moody matriarch and escape the space prison with their lives? Or are they all doomed to fail this diabolical trial by leisure? Galaxy Cruise: Trial by Leisure is the third madcap voyage of the cruise starship WTF Americano Grande , featuring nonsensical sporting events, parasitic potatoes, and a fish playing mind games with a plant. Join the adventure today!

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Galaxy Deals

Are you looking for a cruise that allows you to connect with the Galapagos Islands on a deeper level while also pampering you with first-class amenities and service? Look no further than the M/C Galaxy I Galapagos cruise. With nine cabins hosting up to 16 guests, this 114-foot ship can be your home for the adventure of a lifetime.

Life On Board

An M/C Galaxy cruise expertly blends comfortable private places for relaxation and rejuvenation with social areas for sunbathing and meeting fellow travelers. From the air-conditioned cabins to a dining room surrounded by panoramic windows, modern-chic décor with rich wood tones and soft neutral hues sets the stage for comfortable sophistication. To make the most of every moment in this unique part of the world, head to the sun deck where you can recline on a lounge chair under a sunroof as you watch sea life, birds, and ever-changing landscapes.

For some fun away from the sun, curl up on a sofa in the living room, play a board game, sign in to the all-new Mydas App for onboard entertainment such as movies, digital books, and educational material on the breathtaking Galapagos Islands. Thanks to a delicious selection of menu and buffet foods, all prepared by an expert chef, mealtimes are a highlight of each day. Local and international cuisine, including vegan and gluten-free options, are sure to delight.

Exciting Excursions

No matter which areas or islands of the Galapagos call to you, the M/C Galaxy provides an itinerary to match your desire. Choose an itinerary along a southern route and spend five days exploring the islands of Floreana, Española, and San Cristobal. To experience the far-flung island of Genovesa, opt for the four-day itinerary along the northern route. Can’t bear to miss a thing? The eight-day western route is a deep dive into the area and includes stops along with Fernandina, Santiago, and Isabela islands.

What can you do during your time in the Galapagos? The eight-member crew and your bilingual guide will provide the perfect balance between relaxation and stimulating activities. The Galapagos Islands rank among the top 10 places to snorkel globally, and you can experience this thrill at Tagus Cove on Isabela Island, Pinnacle Rock on Bartolomé Island, or at Devil’s Crown off Floreana Island. Kayaking offers an adventurous, eco-friendly way to explore the coastlines while getting up close to species such as sea lions. Guided hikes are an excellent opportunity to see and learn about natural selection and species such as land iguanas, giant tortoises, and flightless cormorants. Best of all, the details are taken care of for you, and equipment such as wet suits, snorkels, and paddle-boards are included.

Safety At Sea

Aboard the M/C Galaxy, your safety and comfort are always of the utmost priority. The onboard team strives to create an ecological trip with a small environmental footprint without sacrificing convenience or luxury. The ship is Smart Planet certified and provides eco-friendly amenities in every room. Plus, there are ample survival rafts, lifebuoys, and life jackets, and other safety equipment on board, so you can cruise with complete peace of mind.

Anyone looking for authentic experiences in the most hidden spots within the archipelago will enjoy an M/C Galaxy Galapagos cruise, including those who want to do it all to those looking for a slightly slower pace. With oft-overlooked amenities like private bathrooms with hairdryers, a 24-hour coffee station, and a bar serving all your favorite cocktails, the M/C Galaxy has mastered the details of upscale cruising.

Itineraries & Prices

All itineraries are subject to change due to seasonal weather conditions (and resultant variations in river and tributary water levels) affecting accessibility to locations. Thus navigation routes, times and excursions may need to be modified at the cruise captain’s or your guide's discretion.

5 Day - Southern Islands

Embarkation

PM:  San Cristobal airport. Reception at the airport. Transfer in. Welcome onboard Galaxy Cruise. Welcome, drink, and lunch. 

Panga ride, swimming, and snorkeling at Lobos Island. Here, we can walk, swim and snorkel among the seals, frigate birds, marine iguanas, lava lizards, and the small endemic Galapagos snake.

Gardner Bay & Suarez Point

AM:  Breakfast early in the morning. Our first visit starts with a wet landing at Gardner Bay. Activities during this visit: short walk, swimming, snorkeling, and observation of seal colony. The sea lion is one of the animals that receive a lot of attention from visitors in the Galapagos: they are curious and playful, but at times aggressive; they are nice and loving, but also lazy. We are going to find many of them relaxing on the beach enjoying the sun in Gardner Bay. In addition to observing these fascinating animals, we will be able to swim with sea lions, sea turtles, a diversity of tropical fish, marine iguanas, etc. while snorkeling. We will return to the ship and sail to Gardner and Osborn Islets. Lunch. 

PM:  Dry landing at Suarez Point. Activities: Walking, sight-seeing. We recommend athletic shoes, a hat or cap, and sunscreen. There is no color more amazing and outstanding in this tropical desert island than that of the blue-foot boobies. These birds have an impressive form of “greeting,” and it is a real thrill to see their mating dance. The blue-footed booby can be seen on several islands, as nesting grounds and food are located along with several coastal locations. The albatross, an aristocratic bird weighing up to four kilograms, can be observed here. This is one of the largest birds of the islands, with approximately 12,000 couples nesting on Espanola Island and a smaller number nesting on Silver Island, near mainland Ecuador. This species is considered endemic to this island.

During our walk, we will observe these fantastic birds while taking in the beautiful landscapes offered by this island. It is important to note that albatrosses will not be seen between January and March (non-breeding season). We will also see colonies of marine iguanas, masked boobies, finches, Española mockingbirds, and seagulls endemic to the islands. Our walk will be along lava rock, where we will be able to see lava tubes. Red mangroves are found here. Delicious snacks and fresh beverages are waiting for you on the ship. Dinner.

Post Office Bay, Cormorant Point & Devil's Crown

AM:  Breakfast early in the morning. Our first visit will be a wet landing at Post Office Bay. Activities: short walk, swimming, snorkeling, explanation of the mail system. Mail Bay is located on the north side of Floreana Island. Captain James Colnett installed an empty barrel to be used as a post office for the whaling fleets that stopped at the Galapagos Islands. You can leave postcards to your friends and relatives here and/or pick up mail to deliver. Back at the ship, delicious snacks and beverages await. 

PM:  Wet landing, snorkeling, and short trek at Cormorant Point. This point is located on western Floreana Island, and it is famous for its dark green beach (due to a large number of olivine crystals). You can see many marine invertebrates here, as well as sea lions, sea iguanas, frigate birds, and pelicans.  

Activities at Devil's Crown include a panga ride and optional swimming. Enjoy a panga ride around a small island, a satellite islet of Floreana Island. Here, we will find a group of rocks that resemble a crown due to the constant erosion caused by the sea. It is one of the better places to snorkel (shallow diving) in and around the crown. Excellent visibility allows guests to admire a great variety of fish and sea animals. Dinner.

Santa Fe & South Plaza Islands

AM:  Wet landing at Santa Fe this morning, as we enjoy hiking, swimming, and snorkeling. At 60m above sea level, this central island is a unique place to visit. It has an incredible bay with turquoise water, protected by a natural barrier of rocks. You will see colonies of seals, while hawks fly overhead. This island is home to one of two species of land iguanas found in the Galapagos. Their main food source is the leaves and fruit of the large, thick cacti found here. Crabs, nocturnal herons, marine iguanas, mockingbirds, and land doves are frequently seen. The bay is an excellent place to swim and snorkel. Marine tortoises, rays, sea lions, and tropical fish are an amazing attraction! Snacks and refreshing beverages are waiting for you aboard the ship. Lunch. Navigate to South Plazas Island. 

PM:  South Plaza is made up of two small islands created by the uplifting of the land. The land iguanas are smaller here; nesting during the warm season. The hike runs along an extraordinary cliff where seabirds nest. There are approximately 1,000 sea lions located on the south side of Plazas Island. Dinner.

Disembarkation

AM:  Breakfast is very early in the morning. Luggage and belongings should be ready for checkout, our last visit, the Charles Darwin Research Station. The Charles Darwin Station is located in the main port on Santa Cruz Island; we will take a private bus to the entrance of the breeding center where we will see tortoises and land iguanas in captivity. This center is the main breeding site on the islands. Animals that were in danger of extinction on various islands are maintained here in captivity. Thanks to the dedicated work of several institutions, some species are being saved. This is where Lonesome George lived, the famous last survivor of the species of giant tortoise found on Pinta Island. 

After your morning excursion, you will be transferred to Baltra Airport for your departure flight.

8 Day - Western Islands

PM:  Arrival at Baltra Island. This island is known for being inhabited only by military members. This island is home to one of our main airports on the Galapagos. Your guide will be waiting for you upon arrival. Transfer to Santa Cruz Island, crossing the Itabaca Chanel. Then, we take our private bus to Puerto Ayora. Once we arrive at Puerto Ayora, we will go directly to the Galaxy Yacht. After lunch, our first visit is to the Highlands of Santa Cruz, where we will go trekking and will find giant tortoises in their natural habitat, which is an amazing natural spectacle. For this visit, we recommend good walking shoes, long pants, a raincoat, hat, and sunblock. Return to Puerto Ayora. In the evening, there will be a welcome cocktail and a formal presentation of the crew by the captain. There will also be a briefing for the following day. Dinner.

Moreno Point & Elizabeth Bay

AM:  Moreno Point is located on the north coast of Isabela Island between Sierra Negra and Cerro Azul Volcanoes. The main attractions at Punta Moreno are coastal lagoons amid black lava flows where there are several species of birds. It has a panoramic view of the three most active volcanoes in the Galapagos: The Sierra Negra, the Cerro Azul, and La Cumbre. Lunch. 

PM:  This is one of the island’s breeding sites for penguins. Located along the west coast of Isabela, Elizabeth Point is a marine visitor site. Visit Red Mangrove Cove. Here you will spot brown pelicans, flightless cormorants, spotted eagle rays, golden rays, and sea lions. Dinner. 

Urbina Bay & Tagus Cove

AM:  Urbina Bay is located at the base of Alcedo Volcano, between Elizabeth Bay and Tagus Cove. Wet landing. Trail length is around 3.2km; during this walk, we will observe land iguanas, hawks, and tortoises. It is also possible to see red and blue lobsters. Lunch.

PM:  Tagus Cove is located to the west of Darwin Volcano on Isabela Island. While walking, we will find several inscriptions (names of pirates) dating back to the 1800s. From here, we can see Darwin Lake, a saltwater lake with a depth of approximately 9m. It is very common to see a variety of bird species. Dinner.  

Espinoza Point & Albemarle Point

AM:  Fernandina is the third largest island in the Galapagos and has only one visiting site: Espinoza Point, famous for the large colonies of marine iguanas and the home of the flightless cormorant, the Galapagos penguin, snakes, and more. From Espinoza Point, we can admire the beauty of Fernandina Island and its volcano, which last erupted in May of 2005. Fernandina is the only island that doesn’t have mammals, and the open areas are extremely fragile. January to June is the nesting period of the marine iguana, and as such we take great care while walking here. Return to Galaxy. Lunch.

PM:  Albemarle Point is located to the north of Isabela Island. This visiting site is composed of two visiting areas: the first, a mangrove area between lava flows Pahohoe (lava solidified in a wavy or of accordion) and that form coves and channels. This place is ideal for a panga ride. The second point of the visit is the old radar base left by Americans during World War II near the beach.

The marine iguanas of Punta Albermarle and West Isabela are the largest in the Archipelago. It is an ideal place for a panga ride, where you will find non-flying Cormorants, marine iguanas, seabirds, and various marine species. Dinner.

Egas Port, Espumilla Beach & Buccaneer Cove

AM:  At Egas Port, we find a beautiful black sand beach; to the south of the beach is Pan de Azucar Volcano, whose volcanic deposits have contributed to the formation of the black sand beach. The crater of this volcano contains a saltwater lake that dries up during hot weather. Between 1928 and 1930, explorations to develop a salt mine were carried out, but it was decided that a mine was not feasible. The workers’ use of endemic woods also damaged the environment and the development was scrapped. Descent down into the salt mine is not allowed. Return to Galaxy. Lunch.

PM:  The main attractions this afternoon include Palo Santo wood, the beach, and the stunning landscape; the beach is a very important site for tortoises, as they use it for nesting. At one point, pigs were the main predators of tortoises’ eggs; thus, pigs were eradicated with the creation of the Isabela Project.

Dragon Hill & Black Turtle Cove

AM:  Dragon Hill visitor site is located in the northwestern region of Santa Cruz Island. It consists of a 1,600 m long trail running through 3 different environments. In 1975, it was one of the only places on Santa Cruz Island where healthy land iguanas could be found. That same year, the Galapagos National Park and the Charles Darwin Foundation initiated a program to conserve land iguanas. The lagoons found here contain shrimp, the food of choice of the flamingos. During certain times of the year, shrimp are more abundant, and therefore the flamingo population increases. Lunch. 

PM:  After lunch, our first visit is to Black Turtle Cove, which is situated in the northern region of Santa Cruz. This bay is surrounded by mangroves and is accessible only by dinghy. The shallow cove is a safe refuge for marine life. Black-tip reef sharks, marine turtles, and a variety of rays are often spotted here. Dinner.

Rabida Island & Chinese Hat Islet

AM:  The visitor site, located on the east coast of Rabida Island, consists of a red sand beach, a coastal lagoon behind the beach, and a loop trail approximately 1.1 km in length. The red color of the rocks and sand on the beach is due to the interaction of the very porous volcanic material with environmental factors. Rain, saltwater, and the sea breeze act as oxidizing agents creating a red color. The scenery, arid zone vegetation, and the presence of native and endemic species add to the beauty of this island. Lunch 

PM:  A small islet located near the southeast coast of Santiago. It’s shaped like a Chinese hat when seen from afar. It is an island consisting of a cone type 'splatter' that forms the summit and many lava tubes that run down to the coast. On the west side, you can see pillow-type lava formations, which are an indicator that the flows were formed under the sea and have raised upward, and therefore coral heads are found on the lava. This visit provides an excellent opportunity for the interpretation of geological features, such as lava tubes and lava flows. The trail is 700 m (round trip) and the minimum time it takes for this trek is 30 minutes.

AM:  Dry landing. Seymour was formed by the lifting of volcanic marine lava. Marine fossils dating back to the Pleistocene period are found on this island. In 1932, Captain Alan Hancock and his crew took 72 malnourished land iguanas from Baltra Island to North Seymour Island so that the iguanas could live in better conditions. In 1934, colonists verified that the iguanas were in good condition. The vegetation of North Seymour is bushy and is host to nesting Real Frigate birds of the Galapagos.

8 Day - Northern & Southern Islands E+A

AM : Arrive at Baltra Island. Reception at the airport. Transfer in. Welcome onboard Galaxy Cruise. Welcome, drink, and lunch. 

PM : This islet is located between Baltra Island and North Seymour. Here, we find the largest sea lion colony; it is also possible to observe several species of birds. Enjoy the white sand beach. Hiking. Dinner.

Sullivan Bay & Bartolome Island

AM:  Wet landing and hiking at Sullivan Bay. All roads and the island are composed of lava flows that are still virtually without erosion, covering an extensive area, including lava pahoehoe or cords, and hornitos. Lava Colonizers. Snacks and refreshing beverages await you. Lunch.   

PM:  Dry landing, wet landing for the beach. Activities on this island include walking, swimming, snorkeling, and photography. We will follow a path to the top (114m) where we can observe pioneering plants, such as tiquilia, chamaesyce, and scalesia. The island has been formed by cones and lava pipes where lava once flowed. Following the wet landing at the beach, we will take a path through mangrove swamps and dune vegetation, to the south beach. At the north beach, guests can enjoy swimming and snorkeling. From the top of Bartolome, we will be able to see the islands of Santiago, Rabida, and Isabela. We will see penguins in the Rock Pinnacle. Dinner.

Barranco & Darwin Bay

AM:  Dry landing, activities: hiking. Also known as Prince Phillip's Steps. Here, we will take a walk along the Barranco path. During our journey, we will observe tropical birds, petrels, common frigate birds, red-foot boobies, masked boobies, doves, and finches. Return to Galaxy. Lunch  

PM:  Wet landing, coral beach, activities for this day: short walks, photography, swimming, snorkeling. Genovesa is a small island in the northern part of the Galapagos. Our landing in Darwin Bay will be wet. Near the beach is a mangrove swamp where we will find a colony of frigate birds, marine iguanas, red-footed boobies and masked boobies, the great terrestrial finch, seagulls, herons, and cactus finches. Following the land visit, our passengers can enjoy a swim in the bay. Snacks and refreshments are available. Dinner.

Interpretation Center & Lobos Island

AM:  Dry landing. The Interpretation Center is located in the main port. After breakfast, our first visit is to the Interpretation Center of San Cristobal. This modern and sophisticated architectural space was designed for the natural interpretation of the Galapagos. You will learn about the volcanic formation of the islands, the evolution of species, the history of human settlement, and complex systems and management models. 

PM : Panga ride, swimming, and snorkeling at Lobos Island. Here, we can walk, swim and snorkel among the seals, frigate birds, marine iguanas, lava lizards, and the small endemic Galapagos snake.

Accommodations

Social areas.

Suites & Cabins

Lower / main deck stateroom.

The suite features an array of entertainment from WIFI/internet services, DVD, Blu-Ray, and an iPod dock. It also has private access to an outdoor lounge and private ensuite facilities equipped with all the essential amenities to ensure you have a relaxing shower.

Upper Deck Stateroom

Guests can relax and unwind as they enjoy the majestic scenery. Guests will also find a DVD/ Blu-Ray player, a Bose SoundDock, and HD TV to keep them entertained in their rooms. The suite comes with private bathrooms equipped with amenities for guests’ convenience. It accommodates either a Double Bed or Twin Beds plus an extra bed for a third person, making them convenient for close friends or family members.

Technical Information

• Air conditioning
• Lecture room
• WIFI available
• Kosher Meals
• Snorkeling gear
• Stand up paddleboard
• Kayaks / Canoes

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Flight Discount of $200 - Bonita, Alya, Galaxy & Eco Galaxy

Get a special discount of $200 on local flights on selected departures in 2023 on the 4, 5, 6, 7 and 8-day itineraries!

Free Domestic Flights - Bonita, Alya, Galaxy & Eco Galaxy

Free domestic flights to the Galapagos Islands when you cruise on board the Bonita Yatch, Alya, Galaxy or Eco Galaxy!! This offer applies to select departures.

• I'd like to be contacted via WhatsApp.
• I'd prefer not to be contacted by phone or SMS.
• All non-charter prices displayed are PER PERSON
• Minimum age to emark is 6 years old (on the cruise departure date)
• Kids under 12 years (11 years and 11 months) get 10% discount when sharing a triple cabin with their parents
• Third person/adult in triple occupancy pays the full rate, 10% discount is only for kids under 12
• Groups of 7 passengers minimum, get a discount of 3% (not combinable with other offers)
• High season supplements may apply for Christmas & New Year's period, please note the Single Supplement also increases during high season
• A special fee (of $60 per person) applies for Galapagos air tickets not issued by the operator

Galaxy Cruise Ship, Galapagos

• 9.3 Superb 12 Verified Reviews
• Free Internet
• from ₽ 40,649 / day

Excellent food and drinks “We loved this trip. Every day we snorkeled, went on a tender ride to explore the coastline, and went on a hike/walk to explore the…” Catherine C, United States

• 'Smart Voyager' certification for environmental commitment
• Highly trained crew and naturalist guides
• Explore famous islands with diverse wildlife

The Galaxy is a first-class yacht offering exciting cruises to the Galapagos Islands. She has the coveted 'Smart Voyager' certification, ensuring a commitment to reduce negative impacts on the sensitive Galapagos environment. The crew and naturalist guides are trained to the highest standard to offer a personalized service and unforgettable experience. Social areas include a spacious sun deck with loungers which is the perfect place to enjoy the stunning landscapes, outdoor dining room, indoor lounge, bar, library, and dining room. An assortment of table games, videos, and books are available for your enjoyment, and a variety of sodas, cocktails, and liquors are available for an extra cost at the bar.

Nine well-appointed cabins are available, each with private bathrooms with hot/cold water showers, air-conditioning, a hairdryer, internal telephones, and a safety deposit box. Two cabins are on the lower deck, 3 on the main deck, and 4 on the upper deck. Cabins can be configured to either 2 single or 1 double bed.

Galapagos cruises aboard the Galaxy yacht are available as 4, 6, 8, or 15-day itineraries. Explore the famous islands of Floreana, Santa Fe, Isabela & Santiago to get up close with tortoises, penguins, sea lions, and various colorful birds. Activities include swimming, kayaking, hiking, and snorkeling - snorkel gear is included.

Join the Galaxy yacht for a memorable cruise in the Galapagos Islands. Book online today to reserve your space.

Choose your departure date adventure cruise | snorkelling only

Included: VAT, Fuel Surcharge, Port Fees, Airport Transfer, Drinking Water, Tea & Coffee, Welcome Cocktails, Full-Board Meal Plan (All meals), Snacks, Kayaks, Land Excursions, Naturalist Guide, Snorkeling Guide, Beach Towels, Cabin Towels, Complimentary Toiletries, WiFi internet.

Required Extras: Domestic Flight Non-Issuance Fee (60 USD per trip), National Park Fees (100-200 USD per trip), Visas and Fees (20 USD per trip).

Optional Extras: Gratuities (20-25 USD per day), Local Flights, Alcoholic Beverages, Soft drinks.

Book now, pay later: You can easily place your booking online. We will then hold the spaces for you and you can confirm with a payment later.

* Extra fees are shown per person.

Pay by bank transfer or online with Best Price Guaranteed

Boat features

• Daily housekeeping
• Audio & video entertainment
• Air Conditioned saloon
• Aircon Cabins
• Indoor Saloon
• Non-Diver (Snorkeler) Friendly
• Warm Water Showers
• Smart Voyager accreditation
• En-Suite bathrooms
• Non-Smoking Rooms
• Seaview Cabins
• Snorkeling Equipment
• Naturalist Itineraries
• Dedicated Local & International Crew
• Naturalist Guide
• Leisure Deck
• Observation Deck
• Family Friendly Cruise

Food & Drinks

• Vegetarian Options
• Buffet style
• Beer available
• Vegan Options
• Set Menu Dinner
• Welcome Cocktails

If you have any specific dietary requirements whilst on-board your cruise, you can add this to your special requests on step 2 of the booking form. We advise limiting these requests to religious beliefs or food allergies.

Languages Spoken

The crew speaks English, German, French, and Spanish.

Gear Rental

There is equipment available to rent on this boat. Please provide your rental needs on the booking form. You will need to pay for the rental gear on board of the boat. Show prices .

Drawings & Vessel Layouts

Accommodation.

Upper Deck Cabins

Main Deck Cabins

Lower Deck Cabins

Boat specifications.

• Year built 2007
• Cruising speed 12 Knots
• Engines 2 Caterpilar Motors 440 HP
• Max guests 16
• Number of cabins 9
• Number of bathrooms 9
• Water capacity 4.000 gallons
• Fuel capacity 6.000 gallons

Boat Navigation and Safety

• Radio VHF/DSC/SSB
• Emergency Rafts
• Fire Alarm & Fire Extinguishers
• First Aid Kits
• Satellite & mobile phones
• Search light
• Crew trained in first aid

Galaxy Reviews

• 12 Verified Reviews
• Activities 9.3
• 10 Exceptional
• Catherine C

Planned perfectly

We loved this trip. Every day we snorkeled, went on a tender ride to explore the coastline, and went on a hike/walk to explore the land. The days were perfectly planned. There were snacks after every activity and the crew was super vigilant about safety and keeping us informed.

• 8.0 Very good

Absolutely incredible and utterly unique

A Galapagos cruise seems pricey at first. But once you've experienced one, you will never regret a penny. It's not just that the islands and wildlife are utterly incredible and unlike anything else in this world, there is so much included in that price -- 3-4 excursions a day, zodiacs taking you to and from the islands, snorkeling gear, room + board, nearly a ratio of 1 crew to each guest, as well as the cost of the domestic flight to the islands. It takes an immense amount of effort and resources to execute these trips. It's impossible to describe in words how unique the islands and wildlife are -- it leaves you with a new appreciation of how precious and marvelous our planet it. Just go -- you won't regret it.

Amazing adventure with lots of wildlife

Wide variety of wildlife

Amazing - no two days were the same!!

Swimming with sharks and manta rays

• 8.8 Fabulous
• Catherine S

Great Time on the Galaxy

The opportunity to snorkel with turtles and variety of fish. Our group was multi-generational and the children added a fun dimension to the tours.

A trip filled with a lot of activities, good food and service.

Some of the highlights of the trip are snorkeling, hiking, kayaking, having interesting conversations at the dinner table.

An incredible Galapagos cruise!

My husband and I loved our 5 day/4 night Galapagos cruise onboard the Galaxy. Everything about the cruise was fantastic. The boat was clean and well furnished. The cabins were spacious and comfortable. The features like shower and air conditioning worked well. The food was wonderful and we never went hungry. We particularly liked the snacks they would serve every time we returned to the boat from an activity like snorkeling or beach walk. The crew was kind and professional. The naturalist was spirited and knowledgeable. Everything happened on a precise schedule that was easy to follow (and nice to have structure). The snorkeling was amazing, especially with the high quality scuba mask and wet suits provided. We will absolutely be returning to the Galapagos another cruise!

The crew and guide provided us with a great adventure.

As a bird photographer I was able to get many good photos and was thrilled with a close up of of a Galapogas Hawk. I also took many exciting scenes. The crew were well trained and very friendly and helpful.

Bucket list, check!

Care and concern demonstrated by crew always going the extra mile to keep us smiling. Boat condition and cleanliness was exceptional.

birds watch

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There is equipment available to rent on this boat. Please provide your rental needs on the booking form. You will need to pay for the rental gear on board of the boat.

• Fins Included
• Mask Included
• Wetsuit 10 USD per day

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• 1 double or 2 single beds (amenable twin)
• Aircon with control
• Ensuite Bathroom
• Max 2 guests

4 Cabins located on the upper deck. Each cabin has private facilities: private bathrooms, cold/hot water; air-conditioned, a hair dryer, a telephone for internal communication on board, a safe box, and a high speaker.

3 cabins on the main deck. Each cabin has private facilities: private bathrooms, cold/hot water; air-conditioned, a hairdryer, a telephone for internal communication on board, a safe box, and a high speaker.

2 Cabin located on the lower deck. Each cabin has private facilities: private bathrooms, cold/hot water; air-conditioned, a hairdryer, a telephone for internal communication on board, a safe box, and a high speaker.

Rental Gear Prices

Enquiry - charters and groups.

An iconic expedition

About Galaxy

Live a unique experience and explore the Galapagos Islands aboard our renewed Galaxy Yacht Cruise, exclusivity and personalized service for your dreamed trip while exploring the Enchanted Islands.

Key Features

16 passengers, 1 bilingual guide.

Located on the main deck at the back of the boat, for your entertainment on board, the lounge has a TV, DVD player, small library and toy library. The windows give a beautiful light and a very appreciable view of the sea.

Matrimonial Cabin

Cabin size: 15m 2 Located on the upper deck, ideal for couples or friends, there are 4 double cabins convertible into a matrimonial one, equipped with large windows that offer beautiful light and a spectacular panoramic view of the ocean.

Delve into the multifaceted richness of Ecuadorian gastronomy, exquisitely accentuated with native ingredients from the Galapagos Islands, ingeniously amalgamated with global culinary accents to invigorate your senses.

Our chefs brilliantly manipulate local products to cater to a myriad of dietary preferences. Be it pescetarian, vegan, vegetarian, or gluten-free, we assure you a memorable gastronomic journey tailored to your liking.

Enjoy one of the most exhilarating and interesting activities in the Galapagos, which is snorkeling, which will allow you to explore and discover its beautiful rich marine fauna and flora. We provide you with complete equipment for this, fins, mask and snorkel as well as long and short wetsuits, included in the price.

Itineraries

Itinerary a, technical specifications and safety features.

Safety Equipment

• 6 lifebuoys
• 2 survival rafts, 15 passengers each
• 30 life jackets for passengers
• 20 life jackets for crew
• Flare gun, smoke signals, flashlights with SOS signals and other types of visual signals.
• CO2 extinguishers
• Fire detectors and complete fire protection system
• Fire extinguishers
• External automated defibrillator
• Medicine cabinet

The Sun Deck has a wide Solarium equipped with Lazy Chairs.

On the Upper deck we have 4 cabins and the bridge.

On the main deck, besides the 3 cabins, we have the Dining room with the coffee station, as well as the Living room and a Boutique Briefing area.

On the lower deck, there is the kitchen, one twin cabin, the guide and crew cabins and the dining room for them.

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