STRUCTURAL PARAMETERS OF FAINT GALAXIES FROM PREREFURBISHMENT HUBBLE-SPACE-TELESCOPE MEDIUM DEEP SURVEY OBSERVATIONS

We present a statistical study of the properties of faint galaxies fro m a large catalog of about 13,500 objects found in 112 random fields o bserved with the Hubble Space Telescope Wide Field Camera before refur bishment. The majority of the objects are faint galaxies with 18 < I < 25 mag. For each object we determine half-light radius, magnitude, co lor, and a crude classification based on the object's luminosity profi le, by fitting two-dimensional models to undeconvolved images. The cat alog is better than 80% complete to a depth ranging between I similar to 21 mag for the shortest exposures (0.5 hr), up to 23.8 for the long est exposure (about 5 hr). Number counts in the range 18 < I < 22 mag exceed ground-based results by about 50%; many galaxies in this magnit ude range are small enough that they could be misclassified as stars f rom the ground. A clear bifurcation is found in the observed colors of faint spirals and ellipticals, with the latter becoming significantly redder for I > 20 mag. Typical half-light radii of faint galaxies are somewhat smaller than spirals. Half-light radius is not correlated wi th (V-I) color for I less than or similar to 21 mag; at fainter magnit udes, a weak correlation in the sense of larger galaxies being redder is marginally detected.