The dependence of star formation activity on stellar mass surface density and sersic index in zcosmos galaxies at 0.5 < z < 0.9 compared with SDSS galaxies at 0.04 < z < 0.08

In order to try to understand the internal evolution of galaxies and relate this to the global evolution of the galaxy population, we present a comparative study of the dependence of star formation rates on the average surface mass densities (Sigma(M)) of galaxies at 0.5 &lt; z &lt; 0.9 and 0.04 &lt; z &lt; 0.08, using the zCOSMOS and Sloan Digital Sky Survey (SDSS) surveys, respectively. We derive star formation rates, stellar masses, and structural parameters in a consistent way for both samples, and apply them to samples that are complete down to the same stellar mass at both redshifts. We first show that the characteristic step-function dependence of median specific star formation rate (SSFR) on Sigma(M) in SDSS, seen by Brinchmann et al., is due to the change over from predominantly disk galaxies to predominantly spheroidal galaxies at the surface mass density log Sigma(Mchar) similar to 8.5 at which the SSFR is seen to drop. Turning to zCOSMOS, we find a similar shape for the median SSFR-Sigma(M) relation, but with median SSFR values that are about 5-6 times higher than for SDSS, across the whole range of Sigma(M), and in galaxies with both high and low Sersic indices. This emphasizes that galaxies of all types are contributing, proportionally, to the global increase in star formation rate density in the Universe back to these redshifts. The Sigma(Mchar) "step" shifts to slightly higher values of Sigma(M) in zCOSMOS relative to SDSS, but this can be explained by a modest differential evolution in the size-mass relations of disk and spheroid galaxies. For low Sersic index galaxies, there is little change in the size-mass relation, as seen by Barden et al., although we suggest that this does not necessarily imply inside-out growth of disks, at least not in this redshift range. On the other hand, there is a modest evolution in the stellar mass-size relation for high Sersic index galaxies, with galaxies smaller by similar to 25% at z similar to 0.7. Taken together these produce a modest increase in Sigma(Mchar). Low Sersic index galaxies have a SSFR that is almost independent of Sigma(M), and the same is probably also true of high Sersic index galaxies once obvious disk systems are excluded.