The Dependence of Star Formation on Galaxy Stellar Mass

We combine Spitzer 24micron observations with data from the COMBO-17 surveyfor ~15,000 0.2 we consider both the ultraviolet (UV) and theinfrared (IR) luminosities, and account for the contributions of galaxies thatare individually undetected at 24micron through image stacking. For allredshifts we find that higher-mass galaxies have substantially lower specificSFR, /, than lower-mass ones. However, we find the striking resultthat the rate of decline in cosmic SFR with redshift is nearly the same formassive and low-mass galaxies, i.e. NOT a strong function of stellar mass. Thisanalysis confirms one version of what has been referred to as `downsizing',namely that the epoch of major mass build-up in massive galaxies issubstantially earlier than the epoch of mass build-up in low-mass galaxies. Yetit shows that star formation activity is NOT becoming increasingly limited tolow-mass galaxies towards the present epoch. We argue that this suggests thatheating by AGN-powered radio jets is not the dominant mechanism responsible forthe decline in cosmic SFR since z~1, which is borne out by comparison withsemi-analytic models that include this effect.Comment: 5 pages, 2 figures, accepted by ApJL; minor corrections are include