A TEST OF GALAXY EVOLUTIONARY MODELS VIA ANGULAR SIZES

The relationship between angular size, magnitude, and redshift of faint galaxies is explored as a potential tool to distinguish between different galaxy evolutionary models. Different models, based on merging, mild luminosity evolution, and no evolution, lead to different predictions of the angular size distribution, redshift-size relation, and magnitude-size relation. The merging model predicts significantly smaller sizes for faint galaxies than the standard model, because of the requirement for more intrinsically small faint objects at high redshift. A dwarf-rich no-evolution model also predicts small sizes for faint galaxies. The mild luminosity evolution model predicts more luminous galaxies of large angular size at high redshift, as does a standard no-evolution model. Prerefurbishment HST Medium Deep Survey observations of magnitudes and sizes of faint galaxies indicate an excess of small versus large faint galaxies, favoring the dwarf rich, no evolution model with respect to the merging model; the other two models are more discrepant with the data. While these results cannot yet rule out with certainty any of the proposed models, they demonstrate the potential of angular size to discriminate between models of galaxy evolution, especially with the high-resolution HST WFPC2 data.