HUBBLE-SPACE-TELESCOPE IMAGING OF STAR-FORMING GALAXIES AT REDSHIFTS Z-GREATER-THAN-3

We present Hubble Space Telescope images of star-forming galaxies at r edshifts z > 3. These galaxies have been selected using ground-based i mages and color criteria sensitive to the presence of a Lyman disconti nuity in the otherwise flat (in f(v) units) UV spectral energy distrib ution of unreddened star formation. The spectroscopic confirmation of these z > 3 galaxies is reported in a companion paper (Steidel et al.) . The HST images, which probe the rest-frame UV between 1400 and 1900 Angstrom show that the morphologies of the z > 3 galaxies are generall y compact, although we find a few cases of more diffuse light profiles and several cases where the objects are comprised of multiple compact structures. Overall, the dispersion of morphological properties is re latively narrow, in contrast to the variety found in star-forming gala xies at intermediate redshifts (z similar to 1). The galaxies with com pact morphology are typically characterized by a small but resolved '' core,'' approximately less than or similar to 0.7 arcsec in radius, or about 5 h(50)(-1) (8.5 h(50)(-1)) kpc with q(0)=0.5 (0.05), and half- light radii of 0.2-0.3 arcsec, or 1.4-2.1 h(50)(-1) These sizes and sc ale lengths are similar to those of present-day bulges or intermediate -luminosity spheroids. The ''cores'' are often surrounded by lower sur face brightness nebulosities, generally asymmetrically distributed. Th e minority of more diffuse galaxies do not possess this core, and an e xponential function provides a very good fit to their light profiles. In contrast to highly elongated or irregular structures, such as ''cha in galaxies'' that are found at z similar to 1, the z > 3 galaxies are characterized by a relatively high degree of spherical symmetry. The morphological properties, space density, star-formation rates, masses, and early epoch of the star-formation phase all support the hypothesi s that we have identified the progenitors of present-day luminous gala xies at the epoch when they were forming the stars of their spheroidal components.