Massive, star-forming clumps are a common feature of high-redshift star-forming galaxies. How they formed, and why they are so rare at low redshift, remains unclear. In this paper we identify the largest sample yet of clumpy galaxies (7050) at low redshift using data from the citizen science project Galaxy Zoo: Clump Scout, in which volunteers classified 58,550 Sloan Digital Sky Survey (SDSS) galaxies spanning redshift 0.02 < z < 0.15. We apply a robust completeness correction by comparing with simulated clumps identified by the same method. Requiring that the ratio of clump to galaxy flux in the SDSS u band be greater than 8% (similar to clump definitions used by other works), we estimate the fraction of local star-forming galaxies hosting at least one clump (f clumpy) to be 3.22-0.34+0.38% . We also compute the same fraction with a less stringent relative flux cut of 3% ( 12.68-0.88+1.38% ), as the higher number count and lower statistical noise of this fraction permit finer comparison with future low-redshift clumpy galaxy studies. Our results reveal a sharp decline in f clumpy over 0 < z < 0.5. The minor merger rate remains roughly constant over the same span, so we suggest that minor mergers are unlikely to be the primary driver of clump formation. Instead, the rate of galaxy turbulence is a better tracer for f clumpy over 0 < z < 1.5 for galaxies of all masses, which supports the idea that clump formation is primarily driven by violent disk instability for all galaxy populations during this period.
Bibliographical noteFunding Information:
This research is partially supported by the National Science Foundation under grant AST 1716602.
This publication uses data generated via the Zooniverse.org platform, development of which is funded by generous support, including a Global Impact Award from Google, and by a grant from the Alfred P. Sloan Foundation.
This material is based on work supported by the National Aeronautics and Space Administration (NASA) under grant No. HST-AR-15792.002-A.
This research made use of Montage. It is funded by the National Science Foundation under grant No. ACI-1440620 and was previously funded by the National Aeronautics and Space Administration’s Earth Science Technology Office, Computation Technologies Project, under Cooperative Agreement No. NCC5-626 between NASA and the California Institute of Technology.
© 2022. The Author(s). Published by the American Astronomical Society.