Title: Galaxy Zoo: Clump Scout: Surveying the Local Universe for Giant Star-forming Clumps
Authors: Dominic Adams, Vihang Mehta, Hugh Dickinson, Claudia Scarlata, Lucy Fortson, Sandor Kruk, Brooke Simmons, and Chris Lintott
First Author’s Institution: Minnesota Institute for Astrophysics, University of Minnesota Twin Cities, Minneapolis, MN, USA
Status: Published in AAS Journals [open access]
“Clumpy” galaxies get their name from their large, bright star forming clumps. These regions have masses of around ten million to one billion solar masses; for reference, our Milky Way weighs about one trillion solar masses. These regions of high star formation outshine their host galaxy, and are important tracers of galactic evolution.
There are two main theories for how clumps form. In the ‘in-situ’ model, clumps form within the galaxy as a result of its own natural motion. The turbulent motion of the gas in the galaxy’s disk can result in instabilities which could grow into these star forming clumps. In the ‘ex-situ’ model, galaxy mergers lead to clump formation. One of the merging galaxies might become a clump itself, or the process might be the trigger needed to lead to the ‘in-situ’ method. The formation method might be related to the galaxy’s mass, with larger galaxies, including our Milky Way (> ten billion solar mass), being more likely to form clumps in-situ, and smaller galaxies forming clumps ex-situ.
Clumps are a common feature in high redshift galaxies, but clumps exist on sub-kiloparsec scales, which makes them difficult to resolve. A low redshift population of clumps would be easier to study, but there is a noticeable decline in the number of clumpy galaxies at low redshifts. Clumps are notably dominant at z~2, occurring in 
Citizen Science
The authors turned to citizen scientists to create the first major catalog of local clumpy galaxies. Citizen science projects ask ordinary people to perform classification and labeling tasks that computers can’t do on datasets that are too big for one poor, underpaid grad student. The most famous project is the original Galaxy Zoo program, which classified millions of galaxies from the SDSS mission.
This project, Galaxy Zoo: Clump Scout, was built on classifications from Galaxy Zoo 2 (GZ2), and ran from 2019-2021. The Clump Scout team limited their sample to GZ2 galaxies with known masses and redshifts between 0.02 and 0.15. This represents a set of galaxies that are far enough away that the clumps look like point sources, but close enough that the state of the universe matches the local universe. They then removed merging galaxies and selected for sources that GZ2 volunteers classified as likely to have spiral arm features or a disk, as spiral galaxies tend to have higher rates of star formation. The addition of ~5,000 low redshift galaxies with no arms or disks filled out the sample, giving them a total of 58,550 galaxies for the study.
Clump identification is highly subjective, especially by non-experts. In order to evaluate the difficulty of identification and extrapolate how many clumps the volunteers missed, the authors created a range of simulated clumps. These clumps were added into half of the galaxy sample, and volunteers were shown a mix of real and simulated galaxy images to help evaluate their performance.
Clump City
The project identified 7,052 low-redshift clumpy galaxies with 10,739 clumps, the largest sample at the time. Once the authors had this new sample, they were able to estimate the percent of local galaxies containing clumps. The simulated clumps in the sample were key here, as it allowed them to estimate how many clumps the volunteers missed. This allowed the authors to correct their fractional estimate for the missed clumps based on the clump’s general brightness, relative g-r color, and brightness relative to the background.
The authors calculated two local clumpiness fractions: one where the clumps were required to be more than 8% brighter than the host galaxy, and one where they were only required to be 3% brighter. The 8% threshold is common in high redshift studies, but might exclude resolvable clumps at low redshifts, so the 3% cutoff was also used. At the 8% cut off they found a local clumpiness rate of 2.68%, and at 3% a rate of 11.33%.
With these new rates, the study was able to compare the local clump percentage to the clump rates at higher redshifts. They found a sharp decline in clumpy galaxies over redshifts of 0 to 0.5 (Figure 3). Two known trends in galaxy evolution in that time period point to the in-situ model as the better explanation for this decline. The rate of minor galaxy mergers (a small galaxy into a larger galaxy) is observed to be roughly constant over that time; if the clumps formed ex-situ then the two should trend downwards together. Galaxy turbulence rates are observed to decline over that redshift range as well as galaxies “settle”. If the clumps form in-situ, then this decline in turbulence pairs with the decline in clump formation. This catalog and study represent a major contribution to our understanding of clumpy galaxies and their cosmological evolution.
While the Clump Scout project is finished, you can check out and participate in other citizen science projects at Zooniverse.
Edited by: Nathalie Korhonen Cuestas
Featured Image Credit: This paper and Karsten Ratzke under CC0.
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