2007ApJ...661L..41Z

We combine Spitzer 24 µm observations with data from the COMBO-17 survey for ∼15,000 0.2<z≤1 galaxies to determine how the average star formation rates (SFRs) have evolved for galaxy subpopulations of different stellar masses. In the determination of SFR, we consider both the ultraviolet (UV) and the infrared (IR) luminosities, and account for the contributions of galaxies that are individually undetected at 24 µm through image stacking. For all redshifts, we find that higher mass galaxies have a substantially lower specific SFR, <SFR>/<M_*>, than lower mass ones. However, we find the striking result that the rate of decline in cosmic SFR with redshift is nearly the same for massive and low mass galaxies, i.e., not a strong function of stellar mass. This analysis confirms one version of what has been referred to as ``downsizing'', namely, that the epoch of major mass buildup in massive galaxies is substantially earlier than the epoch of mass buildup in low-mass galaxies. Yet it shows that star formation activity is not becoming increasingly limited to low-mass galaxies toward the present epoch. We argue that this suggests that heating by AGN-powered radio jets is not the dominant mechanism responsible for the decline in cosmic SFR since z∼1, which is borne out by comparison with semianalytic models that include this effect.