A constant characteristic mass for star forming galaxies since z~3 revealed by radio emission in the COSMOS field

TL;DR

This study uses radio emission data to show that the characteristic mass of star-forming galaxies has remained constant since redshift 3, challenging models of galaxy evolution that suggest a shift to lower mass systems over time.

Contribution

It provides the first robust evidence that the characteristic stellar mass for star-forming galaxies remains unchanged since z~3, questioning the downsizing paradigm.

Findings

The specific SFR follows a power-law with stellar mass up to z=3.

No significant evolution in the characteristic stellar mass of star-forming galaxies since z~3.

The results suggest a rapid decline in cosmic molecular gas density over time.

Abstract

We present results of our 1.4 GHz image stacking analysis of mass-selected galaxies in the COSMOS field. From the resulting median radio continuum flux density we have determined the evolution of the average star formation rate (SFR) of galaxies as a function of stellar mass, unbiased from effects of dust but also source confusion due to the 1.5" angular resolution achieved by the VLA. We find a power-law relation between specific SFR (SSFR) and stellar mass for star forming galaxies out to z=3. While higher mass systems exhibit lower SSFRs at any epoch, no differential, more rapid evolution of high mass galaxies is evident at least out to z~1.5 where our conclusions are most robust. Utilizing measured mass functions of star forming systems, the characteristic stellar mass for galaxies contributing most to the comoving SFR density appears not to evolve. These findings hence challenge 'downsizing' scenarios in which star formation has gradually shifted towards lower mass systems with cosmic time. Our analysis constitutes the to-date best determination of the cosmic star formation history (CSFH) since z=3 and yields indirect evidence for a rapid decline of the global mass density of molecular gas with time.