Hubble Space Telescope Imaging of Star-forming Galaxies at Redshifts Z > 3

Abstract

We present Hubble Space Telescope images of star-forming galaxies at redshifts z > 3. These galaxies have been selected using ground-based images and color criteria sensitive to the presence of a Lyman discontinuity in the otherwise flat (in f_nu_ units) UV spectral energy distribution 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 A, show that the morphologies of the z > 3 galaxies are generally 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 relatively narrow, in contrast to the variety found in star-forming galaxies at intermediate redshifts (z ~ 1). The galaxies with compact morphology are typically characterized by a small but resolved "core," approximately <~ 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^(2.4-3.6 h_50_^-1^) kpc. These sizes and scale lengths are similar to those of present-day bulges or intermediate-luminosity spheroids. The "cores" are often surrounded by lower surface brightness nebulosities, generally asymmetrically distributed. The minority of more diffuse galaxies do not possess this core, and an exponential function provides a very good fit to their light profiles. In contrast to highly elongated or irregular structures, such as "chain galaxies" that are found at z ~ 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 hypothesis that we have identified the progenitors of present-day luminous galaxies at the epoch when they were forming the stars of their spheroidal components.