Hubble Space Telescope Imaging of Star-Forming Galaxies at Redshifts z>3

Abstract

We present HST images of star-forming galaxies at redshifts z>3. These galaxies have been color selected for having a Lyman discontinuity in the otherwise flat (in f_\nu units) UV spectra of unreddened star formation. The spectroscopic confirmation of these z>3 galaxies is reported in a companion paper (Steidel et al. 1996). The HST images probe the rest-frame UV at 1400--1900 Ang and show that the morphologies of the z>3 galaxies are generally compact, although we find a few cases of more diffuse light profiles and objects comprised of multiple compact structures. Overall, the dispersion of morphologies 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 (8.5 h_50) 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 (2.4--3.6 h_50) 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 $z \sim 1$, the z>3 galaxies are characterized by a relatively high degree of spherical symmetry. Morphology, space density, star-formation rates, masses, and 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