Green pea galaxies may have been responsible for re-ionizing the Universe

The “green pea” galaxies were one of the first discoveries of the Galaxy Zoo; they were first noticed by several of our early volunteers, and appeared in a paper led by Carie Cardamone in 2009 (with over 100 citations so far!). They’ve been the subject of a great deal of follow-up research since then, much of which we’ve tried to follow on this blog.

Images of Green Peas

Examples of the green pea galaxies originally discovered in Galaxy Zoo. Top Row: Sloan Digital Sky Survey images. Bottom Row: Hubble Space Telescope images.

A new paper on the Green Peas has just appeared in Nature, one of the most prestigious and widely-read journals in science. A truly international team of researchers (working in Ukraine, Czech Republic, Switzerland, France, Germany, and the United States) made observations of one green pea galaxy, known as J0925+1403, using an ultraviolet spectrograph on the Hubble Space Telescope. They were able to measure emission from what astronomers call “Lyman continuum” photons; this is light produced by massive stars that are solidly in the ultraviolet wavelengths.

The reason this is so important and interesting relates to one of the most fundamental steps in the history of the Universe that astronomers know of. The majority of matter in the Universe is hydrogen (formed shortly after the Big Bang), and much of it exists in diffuse clouds between galaxies, which is called the intergalactic medium. We know from observations that almost all of that hydrogen is currently ionized – that means instead of consisting of a neutral atom with one proton and one electron orbiting it, the average hydrogen atom between galaxies has had its electron stripped away from the proton. This is a big difference because neutral atoms interact with light differently than ionized atoms. If the hydrogen between galaxies were neutral, it would absorb much of the light coming from individual stars and galaxies, making a huge difference in our ability to observe distant objects.

It’s been known for years the Universe is currently ionized; however, about 700 million years after the Big Bang, we know that the Universe used to be neutral. That’s pretty well-established — however, there’s a great deal of debate about what caused the sudden reionization. Something must have produced large numbers of photons that traveled into the intergalactic medium and ionized all of the hydrogen fairly quickly. There have been lots of papers proposing different possible sources for this, including dwarf galaxies, active galactic nuclei, quasars, very early and massive stars, etc.


Diagram illustrating how individual galaxies may have re-ionized the neutral hydrogen in the Universe shortly after the Big Bang. From Erb (2016).

This new paper proposes that green pea galaxies could be responsible for re-ionizing the early Universe. The measurements from this paper show that at least one green pea galaxy is actively emitting photons with sufficient energy to ionize neutral hydrogen. Lots of galaxies can create such radiation, but one unique aspect of the peas is that the photons are escaping the galaxy where they’re being formed. Usually they’re absorbed by dust or gas clouds within the galaxy before they can affect the rest of the Universe. This is the first time that it’s been demonstrated to occur for a green pea galaxy.

The paper (Izotov et al. 2016) is available online. Nature has also published a nice summary at a slightly less technical level to accompany the article that I’d recommend – you can read that here. Please post if you have any questions or want to discuss more about what this means. We’re extremely excited that your discoveries are still yielding new and interesting science!

Galaxy Zoo Highlights from 2015

Following on from the excellent summary of the hi-lights in 2015 for the Radio Galaxy Zoo project, here’s a similar post about results from Galaxy Zoo.

This year we collected 4,755,448 classifications on 209,291 different images of galaxies. You continue to amaze us with your collective efforts. Thank you so much for each and everyone of of these classifications.

The year started with Galaxy Zoo scientists at Mauna Kea observing galaxies, and reported in this wonderful series of blog posts by (former) Zooniverse developer Ed Paget.

Ed, Chris, Sandor, and Becky in front of the telescope

Ed, Chris, Sandor, and Becky on Mauna Kea.

We celebrated 8 years of Galaxy Zoo back in July, with this blog series of all things 8-like about Galaxy Zoo.

An 8 for our 8th birthday!

An 8 for our 8th birthday!

Back in May we finished collecting classifications on the last of our Hubble Space Telescope images. At the AAS in Florida this week, Kyle Willett and Brooke Simmons presented posters on the planned data releases for the classifications.

We both launched and finished classifying the first set of images of simulated galaxies from the Illustris Simulation (read more here: New Images for Galaxy Zoo: Illustris and here: Finished with First Set of Illustris Images). We also launched our first set of images from the DECaLS survey, which is using the Dark Energy Camera (New Images for Galaxy Zoo: DECaLS)

We also launched a new Galaxy Zoo side project – Galaxy Zoo Bars (one of the first projects built on the new Zooniverse Project Builder software), measuring bar lengths of galaxies in the distant Universe. The entire set were measured in less than a year, so thank you to any of you who contributed to that, and if you missed it don’t worry, we have plans for more special projects this year.

We launched a new web interface to explore the Galaxy Zoo classifications.

Screenshot of the Visualisation Tool

Screenshot of the Visualisation Tool

Our contributions to the peer reviewed astronomical literature continue. Papers number 45-48 from the team were officially published in 2015. They were:
– Galaxy Zoo: the effect of bar-driven fueling on the presence of an active galactic nucleus in disc galaxies, Galloway+ 2015.
– Galaxy Zoo: Evidence for Diverse Star Formation Histories through the Green Valley, Smethurst+ 2015.
– Galaxy Zoo: the dependence of the star formation-stellar mass relation on spiral disc morphology, Willett+ 2015.

You can access all 48 team papers using your classifications at the Zooniverse Publication Page. Remember that all Zooniverse papers published in the Monthly Notices of the Royal Astronomical Society – which includes most of the Galaxy Zoo papers – are available open access to any reader, and if we happen to publish elsewhere we always make the post-acceptance version available on the

All of our papers include a version of this acknowledgement to our classifiers: “The data in this paper are the result of the efforts of the Galaxy Zoo volunteers, without whom none of this work would be possible. Their efforts are individually acknowledged at” We all hope you all know how grateful we are for each and every one of your classifications.

This year saw publication of the first paper on Hubble observations of Voorwerpje systems accompanied by an HST press release.

Montage of Hubble images of Voorwerpjes

Montage of Hubble images of Voorwerpjes

One of those papers from (mostly) outside the GZ team discussed a rare examples of double radio sources from spiral hosts, something Radio Galaxy Zoo will find many more of: “J1649+2635: a grand-design spiral with a large double-lobed radio source”, Mao et al. 2015.

Another exciting thing about this year has been the number of papers from non team members using the classifications which are now public (see To date almost 300 astronomical papers have been written which cite the original description of Galaxy Zoo (Lintott et al. 2008) and the two data release papers so far (Lintott et al. 2011 for GZ1 and Willett et al. 2013 for GZ2) have 164 and 34 citations respectively. The number of papers in the Astrophysics Data System which contain the words “Galaxy Zoo” (which you can search in ADS Labs) is an astonishing 700 (409 for refereed publications).

These are just some of the high-lights I’ve pulled together. If I’ve missed your favourite feel free to add it in the comments below. All in all it’s been a great year. Here’s to an equally good 2016!

Galaxy Zoo is in Florida!

As happens every winter, a large fraction of the world’s astronomers have migrated to a large convention center to share and talk about every aspect of research, outreach, education, and methods of astronomy. This is the biannual meeting of the American Astronomical Society: this winter, the 227th edition is being held in Kissimmee, Florida.


Conference center: now with alligators!


Several posters and talks will be on new research results from Galaxy Zoo data. These include:

  • Becky Smethurst (Oxford) – giving a talk on her PhD dissertation research of the quenching history of galaxies and the role of active galactic nucleus (AGN) feedback. (119.04D; Tue)
  • Melanie Beck (Minnesota) – a poster on the bivariate mass-size relation of galaxies as a function of morphology (342.38; Thu)
  • Melanie Galloway (Minnesota) – a poster on the redshift evolution of the fraction of red disk galaxies, using GZ: Hubble data (342.40; Thu)
  • Kyle Willett (Minnesota) – a poster on the upcoming release of the GZ: Hubble catalog (342.41; Thu)
  • Brooke Simmons (UC San Diego) – a poster on the release of the GZ: CANDELS catalog (342.42; Thu)

In addition, there are several other abstracts that that use Galaxy Zoo data (although it may not always be the main focus), and several of our core team members (Chris, Kevin, Lucy, etc) are here, but talking about other aspects of their research.

It looks to be an exciting week, and we’ll try to blog and tweet more about the amazing things going on in astronomy right now. Check out more of what’s going on at #aas227!

Radio Galaxy Zoo Highlights from 2015

Happy New Year!  I hope everyone had a relaxing break. Radio Galaxy Zoo had a couple of highlights over the last year with new discoveries that will be out later this year.  Well done everyone!

We now have over 1.45 million classifications and are at 48% complete.

Here are a few of our notable highlights:
  • progress on the giant WAT is continuing to bring up more interesting information including our JVLA data – potentially 3 additional papers;
  • we obtained 4 hours to obtain a spectrum for four of our green DRAGN with the observations scheduled for March 2016; and
  • with all your work, RGZ has discovered over 100 new giant radio galaxies!
We are continuing to work away on the data that keeps coming in.  Keep your eye out for our next few projects:
  • matching of RGZ classifications to SDSS;
  • merging Galaxy Zoo data with Radio Galaxy Zoo data;
  • our observations with the JVLA on the hybrid radio sample is complete with 60 hours of observing time; and
  • we are working with the International Astronomical Union (IAU) to get the RGZ name official.
A big welcome to our new team members:
  • Martin Hardcastle (Hertfordshire)
  • Sarah White (ICRAR/Curtin)
  • Francesco de Gasperin (Leiden)
All of this could not have been accomplished without all of you – big THANK YOU! Looking forward to a great 2016!
Julie, Ivy and the RGZ team

Happy 2nd Birthday, Radio Galaxy Zoo!


Happy 2nd birthday to Radio Galaxy Zoo!

On Radio Galaxy Zoo’s 2nd birthday, we wish to thank you all for achieving more than 1.42 million classifications (~57.2 years of work)!  That is an extra 20 years of classifications just in the last year.  We are nearly at the halfway mark now so we are cheering you all on for the great effort and dedication that many of you have shown.

So what has the science team done in the last year with your classifications?  Our first refereed paper has  been published.

Anna Kapinska’s HyMoRS paper has been submitted for publication and we have another 3  papers on the sidelines in preparation and awaiting submission in the next few months.

And we have also submitted several proposals to conduct follow-up observations of the many interesting systems that you all have found!

Thank you all very much for your support again. We are most grateful for your help thus far. However, we still have more than half of our sample to classify. So we are seeking your help in the coming days, months & year to complete this monumental task.

We wish you all a wonderful holiday period and a great upcoming year!


Ivy, Julie & RGZ team

Finished with the first set of Illustris images!

We’re really excited to report that, with your help, the first batch of galaxy images from the Illustris simulation was finished last week! While we still have plenty of images still available to be classified (both from Illustris and DECaLS), I wanted to explain again how the images are being sorted in Galaxy Zoo and show some of the very early results we’re getting from your classifications.

The galaxies we selected from Illustris were a really big set – after eliminating galaxies which were likely to be too small or dim for accurate visual identification (we did this by filtering on the mass of the galaxies), we had over 110,000 images. In designing this phase of Galaxy Zoo, though, we wanted to try and prioritize the order of the images being shown so that we could do some early science projects along the way, rather than waiting many months for the entire data set to be finished before we started our analysis. This first set of Illustris data included 10,832 images, which were classified a total of more than 430,000 times by Galaxy Zoo volunteers.

One of the main questions we wanted to answer was: “How is the apparent morphology of a galaxy affected by the angle at which it’s viewed?”  This is an important one – for observations of the real Universe, we can’t change the position of our telescope relative to the objects we’re looking at. If a galaxy is edge-on, for example, we’re really limited in being able to determine if there’s a bar, how many spiral arms there are, etc. In Illustris, though, we can change the viewing angle in the simulation! As a result, we might hypothesize that all edge-on disks should be identifiable as spiral or S0 galaxies at all the other viewpoints.

Here’s a quick test I’ve run of that. Using the set of collated classifications that just finished from Illustris, I looked for unique galaxies that were classified as “edge-on disks” from at least 1 of the 4 viewing angles that we have data for. Then I looked at the GZ classifications for the other viewing angles to see if they were still edge-on. Results:


Very close to what we expect! Only about 10% of galaxies had any edge-on classifications; of those, almost all of them are classified as face-on at every other angle (the big bump at N=1 in this plot). The exceptions are where the disk is aligned with two of the virtual cameras — then, we see it as edge-on twice and face-on twice. Since the cameras are oriented like they’re at points of a pyramid with the galaxy at the center, geometry tells us we should expect a typical disk to be edge-on for 0 cameras most of the time, 1 sometimes, 2 very rarely, and never 3 or 4. Just what we see!

Screen Shot 2015-12-14 at 6.35.42 PM

Example of a spiral galaxy in Illustris seen from four different viewing angles (“cameras”) in the Galaxy Zoo images. This is a rare case where the galaxy is seen twice as an edge-on disk (upper right, lower left) and twice as a face-on disk (upper left, lower right).

We’re excited to be starting on the analysis phase and are, as always, extremely thankful for your help.

Go West, Young (?) Astronomer

Sunset in Southern California

Many bargains must be made in pursuit of an academic career, and chief among them is an openness to a nomadic early-career life in exchange for a better chance at staying permanently put somewhere later. Grad students and postdocs move around. Not only do we travel all over the world sharing and discussing our research, but the relatively short duration of postdocs, and the fact that in astronomy doing at least 2 of them is now the norm, means we regularly pull up roots and dash off to live somewhere else. My friends have collectively done postdocs on all continents, including Antarctica. Including places thousands of miles from friends and family; including places where they can neither read nor speak any of the native languages.

In this context, I am so, so lucky. My first postdoc moved me only a medium distance (across just one ocean), and to a place where I could at least understand the words, even if I didn’t always get every nuance of meaning. At Oxford I made lifelong friends and built great collaborations, and I thought the research itself was pretty good, too.

Turns out NASA agrees with me. Last year I applied for and was awarded an Einstein Fellowship, which is an early-career award lasting 3 years, an independent postdoc that can be taken to any institution in the US. They’re very competitive (I had applied the previous year without success), and I was thrilled to be awarded one at my top-choice host institution. My first day was last week.

Here’s what the 2015 Fellows page has to say about my research plans:

Brooke uses a variety of multi-wavelength data, including highly accurate galaxy morphologies from the Galaxy Zoo project, to research the connection between supermassive black holes and the galaxies that host them. This connection appears to exist over many orders of magnitude in black hole and galaxy mass, but its fundamental origin is still a puzzle. As an Einstein Fellow at the University of California, San Diego, Brooke will investigate supermassive black hole growth in the absence of galaxy mergers, using a rare sample of galaxies which have never had a significant merger yet host growing black holes. These active nuclei, selected because their host galaxies lack the bulges which inevitably result from a galaxy merger, provide powerful leverage to disentangle the complex drivers of black hole growth and determine the origin of observed black hole-galaxy correlations.

During my fellowship I’m planning on moving forward with the research we first published in 2013 investigating bulgeless galaxies with growing black holes. That is: it’s Galaxy Zoo research.

Galaxy Zoo research brought me to Oxford, and now it has brought me to California. UCSD is a great place, and I’ve already made some really excellent scientists. UCSD is also part of the Southern California Center for Galaxy Evolution and has access to some of the world’s best telescopes, so the future is full of potential.

For now, though: I wouldn’t be here, watching sunsets from my office, without your contributions to Galaxy Zoo over the years. Thank you.

Galaxy Zoo and DECaLS images featured by NOAO

Just a quick reference to a piece that came out in Currents, the newsletter of the National Optical Astronomy Observatory (NOAO) in the US. They included a short piece on the classifications from the new images in DECaLS, a survey which is being co-led by NOAO staff and carried out at their southern observing site in Chile. This followed a longer piece in their September newsletter on the first data release for DECaLS, which includes more details on the tremendous capabilities of the new survey. Good reads if you have a moment!

Galaxy Zoo now featuring DECaLS data!

A quick update: Galaxy Zoo volunteers have already provided more than 750,000 classifications of DECaLS images. We’re completely done with about a quarter of the first data release, and all the images have enough early classifications that we’re starting on preliminary analysis soon. As always, thanks to everyone for your interest and help!

“Blue stuff” in the Illustris galaxy images

I think the most common question/comment we’ve been seeing for classifiers of the simulated Illustris galaxies is along the lines of: “What’s the blue stuff?”

Image of a synthetic galaxy (AGZ00089n5) from the Illustris simulation, being classified in Galaxy Zoo.

Image of a synthetic galaxy (AGZ00089n5) from the Illustris simulation, being classified in Galaxy Zoo. Blue-ish emission can be seen extending from the lower left to upper right of the center galaxy.

It’s a great question. Let’s talk about it in more detail.

The short answer is that the blue regions are the simulations’ method of reproducing the light emitted by young stars. A star’s lifetime generally scales as a function of its mass – the more massive the star is when it’s first formed, the hotter it is and the faster it burns fuel. Emission from hotter objects will tend to be bluer (ie, produce more photons at shorter wavelengths) compared to less massive stars. These are trends we see in optical images of stars in galaxies, including naked-eye views and composite color images. The exact color depends on the filters being used as well as processing of the images – that’s the difference between images you may have seen of star-forming regions being pink in some images and blue in others, such as those in Illustris.

A couple more specific questions that we’ve received:

What’s causing the blue colors in the galaxies? Are they caused by individual atomic or molecular lines that we can see in the spectra?

Volunteers who worked on the original GZ green peas project might be familiar with the term “nebular emission” – individual, narrow lines caused by ionized or hot gas surrounding stars, or whether they’re the result of the broadband colors of the stars themselves. The GZ-Illustris images use a stellar population model that only computes the broadband colors, due to some issues with unrealistic green images caused by the interaction of the codes that deal with both the emission lines and effects of dust. The model we’re using – based on work by Bruzual & Charlot (2003) – omits the emission lines for that reason. However, we’ve made extensive comparisons of the two sets of images and find that they agree very well for our scientific goals, including the morphology classifications.

A plot of the synthetic spectra for galaxies in the Illustris simulation; each thin horizontal line is the spectrum of an individual galaxy. The most massive galaxies are at the top, while the lowest mass galaxies are at the bottom. Wavelength increases from left to right, or going from bluer to redder colors. The lack of sharp features in this plot (which uses the BC03 model adopted by the Galaxy Zoo images) are a result of excluding the nebular line emission.

A plot of the synthetic spectra for galaxies in the Illustris simulation; each thin horizontal line is the spectrum of an individual galaxy. The most massive galaxies are at the top, while the lowest mass galaxies are at the bottom. Wavelength increases from left to right, or going from bluer to redder colors. The lack of sharp features in this plot (which uses the BC03 model adopted by the Galaxy Zoo images) are a result of excluding the nebular line emission. Figure courtesy P. Torrey (MIT/Caltech).

How should visual morphology classifiers deal with the star-forming regions? Ignore them and look at the underlying stellar populations? Treat them as part of the galaxy? Something else?

This is a tough one.  Many galaxies have the “blobby” star-forming regions but others have nicer looking disk or spiral distributions.  Our analysis suggests is that this is a pretty tight function of the total star formation rate (higher SFR = more realistic looking features).  We suggest that users treat them as part of the galaxy; it might lead to some odd results in lower mass galaxies, but we expect they should trace each other very well for the more massive galaxies. If you see geometry that’s distinctly different from a well-formed spiral disk or elliptical, don’t be hesitant to click the “Anything Odd” or “Other” buttons – that’s one of the simplest ways in which we can measure the unusual effects of the blue regions, given the constraints of our classification scheme.

One option for measuring the effect of the blue blobs is to select "Other" under the "Anything Odd" question.

One option for measuring the effect of the blue blobs is to select “Other” under the “Anything Odd” question.

The distribution of the blue blobs is often disconnected and/or in unusual shapes compared to Sloan. What determines the spatial distribution of the star forming regions?

This results from the extremely discrete sampling of the density of stars in the images.  Stars can only form in “chunks” of about 1 million solar masses, instead of the more typical small clusters and regions that we know exist in the real Universe.  Moreover, these chunks have their light spread over a significant fraction of ~1 kpc (which is pretty big, compared to a typical galaxy radius of ~20 kpc), and so they often won’t look much like real star-forming regions.  This, coupled with the lack of dust, leads to what you see in the GZ images.

Thanks as always to everyone for your help. Please post here or on Talk if you have more questions!

This post was written with the help of researchers Gregory Snyder (Space Telescope Science Institute) and Paul Torrey (MIT/Caltech), who worked extensively on the development of Illustris and the generation of the mock images for Galaxy Zoo.

Searching for “tidal debris” in DECaLS images

This post was written with the help of Sugata Kaviraj, senior lecturer at the University of Hertfordshire in the UK. Sugata has been a member of the GZ science team for several years and is leading our analysis of tidal debris in the DECaLS images.

One of the biggest changes to the questions we’re asking for the new DECaLS images asks users to help us classify galaxies that are either merging and/or exhibiting “tidal debris”. While mergers have been part of Galaxy Zoo since our very first classifications, tidal debris is looking for something more specific, which the new DECaLS data is particularly suited for.

In astronomy, “tides” are a type of force exerted on an object by the effect of gravity. Specifically, it refers to the gravitational force exerted by one body on another – since the force exerted by gravity depends on the distance to the object (specifically, it’s proportional to the inverse square of the distance), the nearer side of the object will feel a stronger force than the farther side of the object. On Earth, the differential force caused (mostly) by the gravity of our Moon acts on the liquid in the oceans, causing the rise and fall in sea levels each day.

Field tidal.svg
Example of how a solid body can be deformed under tidal forces. (“Field tidal” by Krishnavedala. Licensed under CC BY-SA 3.0 via Commons.)

When the object is a solid body (like a planet or moon), tidal forces can strain and compress the body, resulting in internal heat and sometimes driving geological activity. A galaxy isn’t a solid body, but composed of individual stars/gas clouds/dark matter particles bound within its own gravitational field. When a galaxy is subjected to tidal forces, it pulls the galaxy apart, causing irregularities in shape that can take many forms depending on the magnitude and direction of the forces involved.

One of the main reasons we want to identify tidal features specifically is to make better measurements of the merger history of galaxies. A complication is that the event of merging with a galaxy isn’t an instantaneous event – depending on the relative masses, velocities, and orientations of the merging galaxies, tidal forces strip out long tails of stars and gas from the galactic centers.

The Mice (NGC 4676): Colliding Galaxies With Tails of Stars and Gas
The Mice (NGC 4676): colliding galaxies with tails of stars and gas distorted by tidal forces. Source:

Ultimately, the nuclei of the two galaxies will fully merge; if that happened sufficiently far in the past for the orbits of stars to relax, then it’s difficult for observers to determine if an elliptical galaxy today was the result of a merger.

Tidal debris features, however, are longer-lived signals of a merger sometime in a galaxy’s past. If we only asked about mergers, we’d be restricting the sample to galaxies that we’re lucky enough to observe “right in the act” of merging. By identifying the tidal debris as well, we can make a more complete census of galaxies that had a merger at some point in their past. One project that this is critical for is looking at the history of galaxy populations, and trying to figure out whether star formation and/or active black holes might be powered by merger events.

Since tidal debris features can be very faint (ie, having low surface brightnesses), the deeper DECaLS images that we’re currently classifying are much better at picking out these features than SDSS. That’s the main reason we’re focusing on trying to detect them in the current set of images.

Tidal debris can come in many different forms, including extended light, faint shells, dust lanes, or satellites in the process of being assimilated to clearly distorted galaxies which have presumably have had a recent interaction. The features can definitely be faint, but we’d like to ask that when you reach this question, please take a careful second look at the area around the main galaxy and see if you can spot anything. Examples are also available in the help text, and we hope that you’ll discuss features you’re not certain about with the moderators, scientists, and other volunteers in Talk.

Example 2. Faint tidal debris extension is visible to the upper left of the main galaxy. Image courtesy DECaLS/Galaxy Zoo.

Example 1. Faint tidal debris extension is visible to the upper left of the main galaxy. Image courtesy DECaLS/Galaxy Zoo.

Example 3. Faint tidal debris is visible around the main galaxy in a shell-like shape. Image courtesy DECaLS/Galaxy Zoo.

Example 2. Faint tidal debris is visible around the main galaxy in a shell-like shape (see arrows). Image courtesy DECaLS/Galaxy Zoo.

Example 4. Faint tidal debris extension is visible to the left of the main galaxy. Image courtesy DECaLS/Galaxy Zoo.

Example 3. Faint tidal debris extension is visible to the left of the main galaxy (within red ellipse). Image courtesy DECaLS/Galaxy Zoo.


Example 4. A small satellite galaxy is merging into the larger one at the center, being ripped apart and causing tidal debris. Image courtesy DECaLS/Galaxy Zoo.


Example 5. Tidal debris (within the red outline) extending to the top and left of the main galaxy. Image courtesy DECaLS/Galaxy Zoo.


Example 6. Faint tidal debris in a “shell” structure, extending in several directions from the main galaxy (comparable to Example 3). Image courtesy DECaLS/Galaxy Zoo.

Please let us know, here or on Talk, what questions you have. Thanks for your help in tackling a new and interesting scientific problem.


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