HUBBLE-SPACE-TELESCOPE MEDIUM DEEP SURVEY WITH THE WIDE-FIELD AND PLANETARY CAMERA .1. METHODOLOGY AND RESULTS ON THE FIELD NEAR 3C-273

We present results from the Medium Deep Survey (MDS), a Key Project using the Hubble Space Telescope. Wide Field Camera images of random fields have been taken in ''parallel mode'' with an effective resolution of 0''.12 FWHM in the V(F555W) and I(F785LP) filters. The exposures presented here were targeted on a field 6' away from 3C 273, and resulted in similar to 5 hr integration time in each filter. Detailed morphological structure is seen in galaxy images with total integrated magnitudes down to V similar or equal to 22.5 and I similar to 21.5. Parameters are estimated that best fit the observed galaxy images, and 143 objects are identified (including 23 stars) in the field to a fainter limiting magnitude of I approximate to 23.5. We outline the extragalactic goals of the HST Medium Deep Survey, summarize our basic data reduction procedures, and present number (magnitude) counts, a color-magnitude diagram for the field, surface brightness profiles for the brighter galaxies, and best-fit half-light radii for the fainter galaxies as a function of apparent magnitude. A median galaxy half-light radius of 0''.4 is measured, and the distribution of galaxy sizes versus magnitude is presented. We observe an apparent deficit of galaxies with half-light radii between similar to 0''.6 and 1''.5, with respect to standard no-evolution or mild evolution cosmological models. An apparent excess of compact objects (half-light radii similar to 0''.1) is also observed with respect to those models. Finally, we find a small excess in the number of faint galaxy pairs and groups with respect to a random low-redshift field sample.