GEMS: The Surface Brightness and Surface Mass Density Evolution of Disk Galaxies

We combine HST imaging from the GEMS (Galaxy Evolution from Morphologies and SEDs) survey with photometric redshifts from COMBO-17 to explore the evolution of disk-dominated galaxies since z<~1.1. The sample is composed of all GEMS galaxies with Sérsic indices n<2.5, derived from fits to the galaxy images. We account fully for selection effects through careful analysis of image simulations; we are limited by the depth of the redshift and HST data to the study of galaxies with M_V<~-20, or equivalently, log(M/M_solar)>~10. We find strong evolution in the magnitude-size scaling relation for galaxies with M_V<~-20, corresponding to a brightening of ~1 mag arcsec^-2 in rest-frame V band by z~1. Yet disks at a given absolute magnitude are bluer and have lower stellar mass-to-light ratios at z~1 than at the present day. As a result, our findings indicate weak or no evolution in the relation between stellar mass and effective disk size for galaxies with log(M/M_solar)>~10 over the same time interval. This is strongly inconsistent with the most naive theoretical expectation, in which disk size scales in proportion to the halo virial radius, which would predict that disks are a factor of 2 denser at fixed mass at z~1. The lack of evolution in the stellar mass-size relation is consistent with an ``inside-out'' growth of galaxy disks on average (galaxies increasing in size as they grow more massive), although we cannot rule out more complex evolutionary scenarios.