An Explanation for the Observed Weak Size Evolution of Disk Galaxies

Surveys of distant galaxies with the Hubble Space Telescope and from theground have shown that there is only mild evolution in the relationship betweenradial size and stellar mass for galactic disks from z~1 to the present day.Using a sample of nearby disk-dominated galaxies from the Sloan Digital SkySurvey (SDSS), and high redshift data from the GEMS (Galaxy Evolution fromMorphology and SEDs) survey, we investigate whether this result is consistentwith theoretical expectations within the hierarchical paradigm of structureformation. The relationship between virial radius and mass for dark matterhalos in the LCDM model evolves by about a factor of two over this interval.However, N-body simulations have shown that halos of a given mass have lesscentrally concentrated mass profiles at high redshift. When we compute theexpected disk size-stellar mass distribution, accounting for this evolution inthe internal structure of dark matter halos and the adiabatic contraction ofthe dark matter by the self-gravity of the collapsing baryons, we find that thepredicted evolution in the mean size at fixed stellar mass since z~1 is about15-20 percent, in good agreement with the observational constraints from GEMS.At redshift z~2, the model predicts that disks at fixed stellar mass were onaverage only 60% as large as they are today. Similarly, we predict that therotation velocity at a given stellar mass (essentially the zero-point of theTully-Fisher relation) is only about 10 percent larger at z~1 (20 percent atz~2) than at the present day.Comment: 13 pages, 6 figures, accepted for publication in ApJ. Revised in response to referee's comments to improve clariry. Results are unchange