The VLA-COSMOS 3 GHz Large Project: Evolution of specific star formation rates out to $z\sim 5$

Abstract

We provide a coherent, uniform measurement of the evolution of the logarithmic star formation rate (SFR) - stellar mass ($M_{\ast }$) relation, called the main sequence of star-forming galaxies (MS), for galaxies out to $z\sim 5$. We measure the MS using mean stacks of 3 GHz radio continuum images to derive average SFRs for $\sim$200,000 mass-selected galaxies at $z>0.3$ in the COSMOS field. We describe the MS relation adopting a new model that incorporates a linear relation at low stellar mass (log($M_{\ast }$/M${}_{\odot }$)$<$10) and a flattening at high stellar mass that becomes more prominent at low redshift ($z<1.5$). We find that the SFR density peaks at $1.5<z<2$ and at each epoch there is a characteristic stellar mass ($M_{\ast }=1-4\times 10^{10}{\mathrm{M}}_{\odot }$) that contributes the most to the overall SFR density. This characteristic mass increases with redshift, at least to $z\sim 2.5$. We find no significant evidence for variations in the MS relation for galaxies in different environments traced by the galaxy number density at $0.3<z<3$, nor for galaxies in X-ray groups at $z\sim 0.75$. We confirm that massive bulge-dominated galaxies have lower SFRs than disk-dominated galaxies at a fixed stellar mass at $z<1.2$. As a consequence, the increase in bulge-dominated galaxies in the local star-forming population leads to a flattening of the MS at high stellar masses. This indicates that "mass-quenching" is linked with changes in the morphological composition of galaxies at a fixed stellar mass.

48 pages (main paper 25 pages, 15 figures). Accepted for publication in The Astrophysical Journal