At z = 5, the universe is only 890 h50-1 Myr old, corresponding to a look-back time of 93.2% of the age of the universe. Any galaxies observed at these early epochs must necessarily be in their youth, and any information we can obtain on them is of the utmost interest to studies of galaxy formation. In particular, if these galaxies are truly primeval objects forming their first generation of stars, their morphologies can provide constraints on galaxy formation models. If the formation of a galactic spheroid occurs via the monolithic collapse of a protogalactic cloud (e.g., Eggen et al. 1962), then the bulk of its star formation might occur in a small region kiloparsecs in extent; such high-redshift protogalaxies will appear as compact, luminous objects (Lin & Murray 1992). Alternatively, if galaxy formation is a hierarchical process (e.g., Baron & White 1987; Baugh et al. 1998), protogalaxies may appear as a multitude of unresolved subgalactic clumps embedded in a more diffuse gaseous halo.
All of the confirmed z > 4 galaxies in the HDF (Fig. 13) have compact, but resolved morphologies, with deconvolved half-light radii of 0".2 ( 1-2 h50-1 kpc), comparable to the values found for many of the z ~ 3 Lyman-break galaxies (Giavalisco et al. 1996). The sizes are clearly subgalactic, suggestive of the hierarchical scenarios of galaxy formation. HDF 3-951.1, the brighter component of HDF 3-951.0 (z = 5.33 for both), contains substructure with a second "hot spot" approximately 0".12 east of the core, at a projected separation of 0.66 h50-1 kpc. We speculate that this is either a knot of star formation (bright in the rest-frame UV), or evidence of multiple nuclei. The projected proximity of HDF 3-951.2 adds weight to the hypothesis that this is a dynamically bound system, and that we are witnessing a merger event. HDF 4-439.0 at z = 4.54 and HDF 4-625.0 at z = 4.58 also both have multiple components. Lyman-break galaxies at z ~ 3 often exhibit either disrupted morphologies or multiple components (e.g., Giavalisco et al. 1996; Steidel et al. 1996b; Bunker, Moustakas, & Davis 1999). However, HDF 4-473.0 at z = 5.60 shows no evidence of substructure.
Figure 13. Montage of spectroscopically confirmed z > 4 galaxies in the HDF, from the drizzled F814W (I814) images. Note that most of the galaxies are compact, but show evidence of merging activity (interactions), in the form of multiple nuclei, multiple components, and/or extended, low surface brightness tidal tails.