Galaxy Zoo

Galaxy Zoo is an ongoing astronomy-based citizen science project hosted on Zooniverse that recruits people to classify and identify features of galaxies according to morphological classification. Galaxy Zoo is the best-known online citizen-science project and has the most publications from a citizen-science project. Galaxy Zoo inspired the creation of The Zooniverse.

Galaxies broadly fall into two main categories: blue star-forming disk spirals and red and 'dead' ellipticals. Publications from Galaxy Zoo provide details that there are also extremely interesting populations of red spirals and blue ellipticals that have provided key insights into galaxy evolution.

## Origin

Before its launch, there is a catalog of 900,000 galaxies that need its features to be identified from the Sloan Digital Sky Survey (SDSS). The purpose of the Galaxy Zoo project is to answer a variety of scientific questions, to prepare the ground for morphological work using future instruments like the JWST, and to produce samples of morphologically selected high-redshift galaxies for follow-up using instruments like the extremely large telescopes and ALMA.

## History

On July 11, 2007, Galaxy Zoo was launched to the public, and within 24 hours, where 70,000 classifications were done by one hour. By the end of 2007, 50 million classifications by 150,000 participants were made regarding the project. The dataset is made up of a million galaxies imaged by the Sloan Digital Sky Survey, which still provides some of the images on the site today.

## Galaxy Zoo Projects

As of July 2023, the complete list of Galaxy Zoo projects is:

As of July 2023, the complete list of Galaxy Zoo projects is:

##### - Galaxy Zoo 1

Galaxy Zoo 1 is the first alliteration of Galaxy Zoo. Galaxy Zoo 1 provides visual morphological classifications for nearly one million galaxies, extracted from the Sloan Digital Sky Survey (SDSS). Galaxy Zoo 1 invites users to identify ellipticals, edge-on, clockwise, and counterclockwise spirals, or mergers. The first alliteration ran until 2009 for the replacement of Galaxy Zoo 2. The focus of Galaxy Zoo is extending the count of AGN-host galaxies, overlapping galaxies, merging galaxies, green peas, and red spirals. Discoveries like Hanny's Voorwerp, Collections of Green Pea Galaxies, as well as the overlap of galaxies.

##### - Galaxy Zoo 2

Galaxy Zoo 2 is the successor to the original Galaxy Zoo. Galaxy Zoo 2 extends the original Galaxy Zoo classifications for a subsample of the brightest and largest galaxies in the Legacy release, measuring more detailed morphological features. This includes galactic bars, spiral arm and pitch angle, bulges, edge-on galaxies, relative ellipticities, and many others. Galaxy Zoo 2's main focus is on lengths of bars in disk galaxies, and dust lanes in spheroidal galaxies.

##### - Galaxy Zoo Hubble

Galaxy Zoo: Hubble used data from the Hubble Space Telescope to classify images of distant galaxies beyond the reach of SDSS. Images for Galaxy Zoo Hubble were taken from the Advanced Camera for Surveys (ACS) aboard Hubble, including data from multiple Legacy surveys, like AEGIS, COSMOS, GEMS, and GOODS. Galaxy Zoo Hubble focuses on galactic bar evolution.

##### - Galaxy Zoo CANDELS

Galaxy Zoo CANDELS classified the morphology of galaxies in images from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). CANDELS images were taken with both the ACS and WFC3 cameras on the Hubble Space Telescope, focusing on rest-frame optical wavelengths of galaxies at redshifts of 1 < z < 3.

##### - Galaxy Zoo DECALS

Galaxy Zoo DECaLS is currently classifying the morphology of galaxies in images from the Dark Energy Camera Legacy Survey (DECaLS). DECaLS images were taken with the Dark Energy Camera (DECam) on the Blanco 4m telescope, located at the Cerro Tololo Inter-American Observatory. Deeper DECaLS images (r=23.6 versus r=22.2 from SDSS) reveal spiral arms, weak bars, and tidal features not previously visible in SDSS imaging. We have released classifications for 314,000 galaxies to date.

For the first time, Artificial intelligence (Zoobot) learns morphology predictions made using deep learning models. These models are trained on volunteer answers and predict posteriors for what a typical volunteer might say. When tested on galaxies where the volunteers are confident, they are 99% accurate on every question.

##### - Galaxy Zoo UKIDSS

The United Kingdom Infrared Telescope (UKIRT) for the recently-completed UKIDSS project. UKIDSS is the largest, deepest survey of the sky at near-infrared wavelengths, and the typical seeing is close to (often better than) the typical seeing of the SDSS. UKIDSS project is used to answer fundamental questions of galaxy evolution basically in infra-red data.

##### - Galaxy Zoo Cosmic Dawn

Galaxy Zoo Cosmic Dawn uses the Hawaii Two-0 (H20) survey as part of the wider Cosmic Dawn survey, aiming to push the boundaries of extragalactic astronomy out to earlier in the Universe's history. This effort from Galaxy Zoo will therefore also help prepare for the upcoming launch of the Euclid space telescope by the European Space Agency (ESA) in 2023, with the classifications you make now helping to guide what Euclid will observe in more detail with its even higher resolution imaging in visible light and the infrared. The main focus in Galaxy Zoo Cosmic Dawn alliteration collects bright-red galaxies, to give more precise studies of the universe's formation.

##### - Galaxy Zoo Galaxy Zoo JWST

Galaxy Zoo JWST uses 8000 galaxy images from the Cosmic Evolution Early Research Science (CEERS) survey. CEERS’ main goal is to demonstrate, test, and validate efficient extragalactic surveys with coordinated, overlapping parallel observations in a patch of sky that already has a lot of other multi-wavelength data (e.g. from the Hubble Space Telescope). The program is used to study the formation of galaxies, to the surprise, most galaxies that Hubble cannot see are small, irregular blobs that are really faint.

##### - Galaxy Zoo EAGLE

The EAGLE galaxies track how individual galaxies take their shape in a sophisticated simulated universe. EAGLE galaxies will help our understanding of how the physical properties of galaxies translate to what we see through our telescopes. Scientists will learn more about the galaxies in our real Universe and can further harass the power of Galaxy Zoo.

However things changed shortly after the set was launched, and there was a bit of backlash against the GZ team because the set was rather boring without real galaxies. Instead they added images from GZ DECaLS to make it less boring. That returned to normal as you get an EAGLE 25% when classifying.

##### - Galaxy Zoo Illustris

Galaxy Zoo classifies galaxies from the Illustris Project. Illustris is a state-of-the-art simulation of the Universe, led by a large team of researchers in the US, UK, and Germany. Like the EAGLE galaxies, Illustris is simulated galaxies from computers under galaxy formation. Classifications from both Galaxy Zoo 2 and from the new Illustris data, provide vital tests for the output and will be fed back to the science teams in order to improve future versions of these sims.

##### - Galaxy Builder

Galaxy Builder builds components of galaxies to help us understand how they are assembled. Galaxy Builder made use of citizen science to create disc-bulge-bar-spiral photometric models of 198 spiral galaxies, selected using GZ2 and the NASA-Sloan Atlas. This project lets people create a model of a galaxy image, which scientists can use to learn about its structure, and how it assembled over cosmic time. In the future, Galaxy Builder will revive to process galaxies for the TIMBERS/SGA project.

##### - Galaxy Zoo Bar Lengths

Galaxy Zoo Bar Lengths measure lengths, widths, and angles of galactic bars using an interactive interface.

##### - Galaxy Zoo Supernovae

Galaxy Zoo Supernovae uses the Palomar telescope to find supernovae.

##### - Galaxy Zoo Clump Scout

Galaxy Zoo Clump Scout hunts starforming clumps in spiral or irregular galaxies. Over 7,000 galaxies were found to have clumps while 14,000 volunteers examine nearly 60,000 galaxies. First, it is the only clump catalog of its size in which human eyes identified all of the clumps. While some automated methods do exist to identify clumps, they are still improving; human classifications are our best tool for improving them. Second, it is by far the largest clump catalog of its kind covering nearby galaxies. Most large catalogs have focused on the distant universe. The light from very distant galaxies can take billions of years to reach us, so we effectively see these galaxies as if they existed billions of years in the past. Thanks to past surveys, we know that these distant, early-universe galaxies appear to be overflowing with clumps -- but for a long time, we did not have a similarly detailed picture of clumps in nearby, modern galaxies. This is why *Clump Scout* set out to search nearby galaxies for clumps.

##### - Galaxy Zoo 3D

Galaxy Zoo 3D collaborating with MaNGA and SDSS is a project to identify features in galaxies. Galaxy Zoo 3D helps scientists to improve our understanding of how and why galaxies' appearances change over time.

##### - Galaxy Zoo Quench

Galaxy Zoo Quench contains 6,004 galaxies that are classified as peculiar, irregular blobs. The general picture emerging is that low-mass galaxies appear to be able to quench their star formation via multiple pathways: through mergers with other galaxies, feedback from supernovae explosions, and/or feedback from actively accreting central supermassive black holes (known as Active Galactic Nuclei or AGN). In contrast, high-mass galaxies (with their deeper gravitational potential wells) more often appear to require a major merger (a merger with a galaxy of similar mass to its own) accompanied by AGN activity to quench their star formation. The results from this study will provide useful constraints on the role that post-quenched galaxies play in galaxy evolution, and what caused the quenching in these galaxies, and will help improve our understanding of the role of mergers in galaxy evolution.

##### - Galaxy Zoo Mergers

Galaxy Zoo Mergers provide simulations of merging galaxies. It has cataloged thousands of galaxy mergers from the Sloan Digital Sky Survey as well as simulating the stages of a galaxy merger from a pair of galaxies to a shelled elliptical galaxy.

##### - Galaxy Zoo Mobile

Galaxy Zoo Mobile is the only version of Galaxy Zoo that runs on mobile devices. Galaxy Zoo Mobile classifies ring galaxies to show how they have formed. Galaxy Zoo Mobile splits ring galaxies into many different types. (See Ringed Galaxy Classification)

##### - Radio Galaxy Zoo

Radio Galaxy Zoo uses citizen science to locate and identify supermassive black holes and starforming galaxies, usually emitted by radio waves. Radio Galaxy Zoo uses LOFAR to find these objects and can spark theories about the evolution of supermassive black holes (SMBH)

##### - Galaxy Zoo Weird And Wonderful

Galaxy Zoo Weird and Wonderful is a project in the Galaxy Zoo family to find rare and interesting galaxies and make scientists interested in understanding how to search for the unexpected in general. This is especially critical in the current era of big data astrophysics and upcoming large-scale sky surveys such as the Vera Rubin Observatory's LSST. In running projects like this one, we hope we can learn things that will inspire future Zooniverse projects in fields far from astrophysics.

## Volunteers

Galaxy Zoo recruited volunteers to help with the largest galaxy census ever carried out. Opening the project to the general public saved the professional astronomers the task of studying all the galaxies themselves, resulting in the classification of a large number of galaxies undertaken in a shorter time than what smaller research teams would be able to do, classifying 900,000 galaxies in months rather than years if done by smaller research teams. Computer programs and Artificial intelligence had been unable to reliably classify galaxies: several groups had attempted to develop image-analysis programs. An online forum was later set up two weeks after the initial start, partially due to a large volume of emails being sent around, to the point that it was troublesome for those receiving them to process and respond to them. This led volunteers to point out anomalies that on closer inspection have turned out to be new astronomical objects such as 'Hanny's Voorwerp' and 'the Green Pea galaxies'.

The Galaxy Zoo forum became a hotbed for the discussion of the SDSS images and more general science questions. Galaxy Zoo is thus a hybrid, able to do deep analyses of large data sets that are impossible in any other way. It is a magical place, and it feels like coming home at last." The Galaxy Zoo Forum became a read-only archive in July 2014. After seven years online and over 650,000 posts, it continues to generate science.

As of August 2023, there are 76 published articles as a direct result of Galaxy Zoo and hundreds of thousands of volunteers. In previous studies though, it was found that data produced by volunteers was more likely to contain bias or mistakes.

A conference was held between July 11 2017 at St. Catherine's College, Oxford, UK to recognise the tenth anniversary of the start of Galaxy Zoo in July 2007. 125 million galaxy classifications resulting in 60 peer-reviewed academic papers from at least 15 different projects have been made since July 2007. Discoveries include Hanny's Voorwerp, Green pea galaxies, and more recently objects known as 'Yellow Balls'. On the conference Twitter feed, #GZ10, it states that 10 of the 60 papers have over 100 citations within the Astrophysics Data System in 10 years.

## The changes to the morphological classification

In June 2019, citizen scientists through Galaxy Zoo reported that the usual Hubble classification, particularly concerning spiral galaxies, may not be supported, and may need updating. A paper was published regarding the concern of the usual Hubble classification thought that the types of "spiral arms" in such galaxies do not match the standards of star and nebula population, regarding that "SA galaxies are the ones without nebulae and SD galaxies are the ones with less stars and lots of nebulae."

## Major Discoveries, popular studies and articles.

##### Rotation of Spiral Galaxies

In Galaxy Zoo 1, the rotation also called the chirality, of galaxies has been examined in several Galaxy Zoo-related papers. Galaxy Zoo scientists wanted to determine whether spiral galaxies were evenly distributed, or whether an intrinsic property of the universe caused them to rotate one way or the other. When the Science team came to analyze the results, they found an excess of anticlockwise-spinning spiral galaxies. But when the team asked volunteers to classify the same images which had then been reversed, there was still an excess of anticlockwise classifications, delegating that the human brain has real difficulty discerning between something rotating clockwise or anticlockwise. Having measured this effect, the team could adjust for it and established that spirals near each other tended to rotate in the same direction.

That changed when a paper was published regarding how wherever we are viewed, they rotate due to where they rotate. Another paper was published about how disk galaxies rotate in the way the axis of a supermassive black holes' axis was facing, and how they are due to like what happens in a collision, an object experiences a force for a specific amount of time that results in a change in momentum. The result of the force acting for the given amount of time is that the object's mass either speeds up or slows down (or changes direction). Another paper was published concerning the woundness of the spiral arms means how much rotation of a supermassive black hole may equal a change in an object's mass and momentum may affect the rotational period and how long it will rotate to 360 degrees in a matter of time.

#### Red Spirals and Blue Ellipticals

Mainstream astronomical theory before Galaxy Zoo held that elliptical (or 'early type') galaxies were red in color and spiral (or 'late type') galaxies were blue in color: several papers published as a result of Galaxy Zoo have proved otherwise. A population of blue ellipticals was found. These are galaxies that have changed their shape from spiral to oval but still have young stars in them. Indeed, Galaxy Zoo came about through Schawinski's search for blue elliptical galaxies, as near the end of 2006, he had spent most of his waking hours trying to find these rare galaxies. Blueness in galaxies means that new stars are forming. However, ellipticals are almost always red, indicating that they are full of old and dead stars. Thus, blue ellipticals are paradoxical but give clues to star formation in different types of galaxies.

Also, a population of red spirals was found. These have a different evolutionary path from normal spiral galaxies, showing red spiral galaxies can stop making new stars without changing their shape. Using Galaxy Zoo data for their sample, Tojeiro et al. 2013 found 13,959 red ellipticals, 381 blue ellipticals, 5,139 blue late-type spirals, 294 red late-type spirals, 1,144 blue early-type spirals, and 1,265 red early-type spirals. Chris Lintott stated: "These red spiral galaxies had been lurking in the data and no one had spotted them. They were staring us in the face. Now we know that a third of spirals around the edges of some clusters of galaxies are red." He also stated: "These results are possible thanks to a major scientific contribution from our many volunteer armchair astronomers. No group of professionals could have classified this many galaxies alone." A team using the Hubble Space Telescope has independently verified the existence of red spirals.

It is thought that Red Spirals are galaxies in the process of transition from young to old. They are more massive than blue spirals and are found on the outskirts of large clusters of galaxies. Chris Lintott stated: "We think what we’re seeing is galaxies that have been gently strangled, so to speak, where somehow the gas supply for star formation has been cut off, but that they’ve been strangled so gently that the arms are still there." The cause might be the Red Spiral's gentle interaction with a galaxy cluster as the galaxy moves into a denser environment, there’s a lot of gas in clusters as well as galaxies, and it’s possible the gas from the galaxy just gets stripped off by the denser medium it’s plowing into. The thought came from a paper signing that "Ram Pressure Stripping" can lead to the creation of both red spirals and lenticular galaxies (which are blonde in optical color). The paper targets NGC 4440 (a red spiral) and NGC 4526 (a lenticular galaxy) and the elliptical galaxy Messier 87 both have the same characteristics as elliptical galaxies alike according to their appearance. The paper describes that spirals that have star formation and a blonde color to their appearance are most likely to happen before ram pressure stripping, thus proving the theories from the published articles.

#### Dust in galaxies

The properties of Galactic Dust have been examined in several Galaxy Zoo papers. The interstellar medium of spiral galaxies is filled by gas and small solid particles called dust grains. Despite constituting only a minor fraction of the galactic mass (between 0.1% and 0.01% for the Milky Way), dust grains have a major role in shaping the appearance of a galaxy. Because of their dimension (typically smaller than a few tenths of a micron), they are very effective in absorbing and scattering the radiation emitted by stars in the ultraviolet, optical, and near-infrared. Although the interstellar regions are more devoid of matter than any vacuum artificially created on Earth, there is matter in space. These regions have very low densities and consist mainly of gas (99%) and dust. In total, approximately 15% of the visible matter in the Milky Way is composed of interstellar gas and dust.

The study of dust in galaxies is interesting for many reasons. For example, the dimming effects of dust need to be corrected to estimate the total mass of a galaxy from measurements of its light. Standard candles used to measure the expansion history of the Universe also need to be corrected for dust extinction.

A catalog of 1,990 overlapping galaxies was published in 2013, which had been collected by volunteers on the Galaxy Zoo forum using SDSS images. The abstract states: 'Analysis of galaxies with overlapping images offers a direct way to probe the distribution of dust extinction and its effects on the background light.' This catalog was also used in a study of ultraviolet attenuation laws.

#### Galaxy "bars" and "bulge" shape

Some spiral galaxies have central bar-shaped structures composed of stars. These galaxies are called 'barred spirals' and have been investigated by Galaxy Zoo in several studies. It is unclear why some spiral galaxies have bars and some do not. Galaxy Zoo research has shown that red spirals are about twice as likely to host bars as blue spirals. These colors are significant. Blue galaxies get their hue from the hot young stars they contain, implying that they are forming stars in large numbers. In red galaxies, this star formation has stopped, leaving behind the cooler, long-lived stars that give them their blonde color.

Spiral galaxies usually have 'bulges' at their centers. These bulges are huge, tightly packed groups of stars. However, using Galaxy Zoo volunteer classifications, it has been found that some spiral galaxies do not have bulges. Many galactic bulges are thought to host a supermassive black hole at their centers: however pure disk galaxies with no bulges but with growing central black holes were found. That pure disk galaxy and their central black holes may be consistent with a relation derived from elliptical and bulge-dominated galaxies with very different formation histories implies the details of stellar galaxy evolution and dynamics may not be fundamental to the co-evolution of galaxies and black holes. It seems that these bulgeless galaxies have formed in environments isolated from other galaxies. It is hypothesized that the black hole mass may be more tightly tied to the overall gravitational potential of a galaxy and therefore its dark matter halo, rather than to the dynamical bulge component.

In September 2014, a paper titled: "Galaxy Zoo: CANDELS Barred Disks and Bar Fractions" was accepted for publication by the MNRAS. This was the first set of results from the Hubble Space Telescope CANDELS survey that was part of Galaxy Zoo 4. The study reports "the discovery of strong barred structures in massive disk galaxies at z ≈1.5 in deep rest-frame optical images from CANDELS". From within a sample of 876 disk galaxies identified by visual classification in Galaxy Zoo 4, 123 barred galaxies are examined. It is found that the bar fraction across the redshift range 0.5 < z < 2 does not significantly evolve.