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 < z < 0.9 and 0.04 < z < 0.08 , using the zCOSMOS and 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 .
( 8 ) , is due to the changeover from predominantly disk galaxies to predominantly spheroidal galaxies at the surface mass density log \Sigma _ { Mchar } \sim 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 .
( 1 ) , 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 \sim 25 % at z \sim 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 .