The Axis Ratio Distribution of Faint Galaxies: Evidence for a Population of Dwarf Galaxies at I approximately 20.5

Abstract

The axis ratio distribution of faint galaxies observed by the Hubble Space Telescope (HST) as part of the Medium Deep Survey key project suggests that a very large fraction of the total population at magnitudes I >= 20 mag consists of a class of galaxies with luminosity profile, axis ratio distribution, angular size, and color that resemble local dwarfs. We find that galaxies with exponential light profiles and small angular sizes (half-light radius < 0.6") have an axis ratio distribution that is incompatible with their being intrinsically flattened objects and is instead consistent with local elliptical galaxies. We call these objects Small Exponential Ellipticals. They are most likely dwarf galaxies, and they are numerous enough that, together with irregular galaxies, they can account for most if not all Of the excess in the number counts at I ~ 20- 21 mag with respect to the standard no-evolution models. This may suggest that the excess number counts are best explained by dwarf-rich models with strong luminosity evolution of the dwarf galaxies. Our data also supports a very mild luminosity evolution of the giant galaxy populations, which contributes little to the excess number counts.