In the past decade, the Hubble Space Telescope has observed large numbers of distant galaxies. Nonetheless, the process of galaxy assembly and formation at high redshifts remains poorly constrained. There is presently little information on structure formation and star-formation processes within these high-redshift galaxies. This dissertation presents results from three studies in the Hubble Ultra Deep Field (HUDF), the deepest optical data yet, to understand these distant galaxies.

The first part of this dissertation is a study of faint compact Lyman-break galaxies (LBGs) at redshift 4 to 6 (about 1 billion years after the big bang) in the HUDF. These LBGs are too faint individually to accurately measure their radial surface brightness (SB) profiles. The HUDF images of sets of these LBGs, pre-selected to have nearly identical compact sizes and the roundest shapes, were co-added. From these composite images, average SB profiles were then computed that show that even the faintest galaxies at redshift 4 to 6 are resolved and that the inner regions are best represented by disk-like Sersic profiles.

The second part of this dissertation utilizes the deep GRism ACS Program for Extragalactic Science (GRAPES) to spectroscopically confirm 47 LBGs at redshift 5 to 6 in the HUDF. These 47 galaxies are less dusty than galaxies at redshift 3, and their peak star-formation rate (SFR) intensity (i.e., SFR per unit area) does not vary significantly from that in the local universe. The constancy of this peak intensity implies that the same physical mechanisms limit starburst intensity at all redshifts up to 6.

The third part of this dissertation uses the HUDF images and the GRAPES spectroscopy to explore the stellar population ages of the bulges in late-type galaxies (i.e., Hubble types Sb-Sd) at redshift one. The results show that these late-type bulges are younger and less massive than bulges in early-type galaxies at similar redshifts, and that these late-type bulges are better fit by an exponential than by de Vaucouleurs SB profile. The overall picture emerging from this analysis is that, in late-type galaxies at redshift one, bulges form from disk material rather than from a major-merger event.