The Morphology of Faint Galaxies in Medium Deep Survey Images Using WFPC2

Abstract

First results from HST Medium Deep Survey images taken with WFPC2 demonstrate that galaxy classifications can be reliably performed to magnitudes I_814_ <~ 22.0 in the F815W band. Published spectroscopic surveys to this depth indicate a mean redshift of z^bar^ ~ 0.5. We have classified over 200 galaxies in nine WFPC2 fields according to a basic morphological scheme. The majority of these faint galaxies appear to be similar to regular Hubble-sequence examples observed at low redshift. To the precision of our classification scheme, the relative proportion of spheroidal and disk systems of normal appearance is as expected from nearby samples, indicating that the bulk of the local galaxy population was in place at half the Hubble time. However, the most intriguing result is the relatively high proportion (40%) of objects which are in some way anomalous, and which may be of relevance in understanding the origin of the familiar excess population of faint galaxies established by others. These diverse objects include apparently interacting pairs whose multiple structure is only revealed with HST's angular resolution, galaxies with superluminous star-forming regions, diffuse low surface brightness galaxies of various forms, and compact galaxies. These anomalous galaxies contribute a substantial fraction of the excess counts at our limiting magnitude, and may provide insights into the "faint blue galaxy" problem.