An Explanation for the Observed Weak Size Evolution of Disk Galaxies
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Abstract
Surveys of distant galaxies with the Hubble Space Telescope and from the ground have shown that there is only mild evolution in the relationship between radial 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 Sky Survey (SDSS), and high redshift data from the GEMS (Galaxy Evolution from Morphology and SEDs) survey, we investigate whether this result is consistent with theoretical expectations within the hierarchical paradigm of structure formation. The radius-mass relation for virialized dark matter halos in the concordance LCDM model evolves by about a factor of two over this interval. However, high resolution N-body simulations have shown that dark matter halos in hierarchical models build up from the inside out, so that the inner part of the halo, where the baryons are concentrated, changes very little over this interval. We compute the expected disk size-stellar mass distribution, accounting for this evolution in the internal structure of dark matter halos and the adiabatic contraction of the dark matter by the self-gravity of the collapsing baryons. We find that the predicted evolution in the mean size at fixed stellar mass since z~1 is about 15-20 percent, in good agreement with the observational constraints from GEMS. At redshift z~2, the model predicts that disks at fixed stellar mass were on average only 60 percent as large as they are today. This is somewhat stronger evolution than the available observations indicate, but is consistent with the data within the uncertainties.