Galaxy Zoo: Clump Scout: Surveying the Local Universe for Giant
  Star-forming Clumps
  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 yet sample of
clumpy galaxies (7,052) at low redshift using data from the citizen science
project \textit{Galaxy Zoo: Clump Scout}, in which volunteers classified over
58,000 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 galaxies
hosting at least one clump ($f_{clumpy}$) to be $2.68_{-0.30}^{+0.33}\%$. We
also compute the same fraction with a less stringent cut of 3\%
($11.33_{-1.16}^{+0.89}\%$), as the higher number count and lower statistical
noise of this fraction permits sharper 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.
