  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.
