The assembly of diversity in the morphologies and stellar populations of high-redshift galaxies

We have used deep images from the Hubble Space Telescope to study the evolution in the morphologies, sizes, stellar masses, colors, and internal color dispersion of galaxies in the Hubble Deep Field-North. We selected two galaxy samples at 0.7 <= z <= 1.4 and 1.9 <= z <= 3.0 from a near-infrared, flux-limited catalog with complete photometric and spectroscopic redshift information. At z similar to 1 the majority of galaxies with M(B) <= -20.5 have rest-frame optical morphologies of early-to mid-type Hubble-sequence galaxies, and many galaxies show strong transformations between their rest-frame UV and optical morphologies. In stark contrast, galaxies at z similar to 2.3 all have compact and irregular rest-frame optical morphologies with little difference between their rest-frame UV and optical morphologies, and with no clearly evident Hubble-sequence candidates. The mean galaxy size increases from z similar to 2.3 to z similar to 1 by roughly 40%, and the number density of galaxies larger than 3 kpc increases by a factor of approximate to 7. The size-luminosity distribution of z similar to 1 galaxies is broadly consistent with that in the local universe, allowing for passive evolution. However, we argue that galaxies at z similar to 2.3 are not the fully formed progenitors of presentday galaxies, and they must continue to grow in both their sizes and stellar masses. We have quantified the differences inmorphology by measuring the galaxies' internal UV-optical color dispersion, which constrains the amount of current star formation relative to older stellar populations. The mean and scatter in the galaxies' UV-optical total colors and internal color dispersion increase from z similar to 2.3 to z similar to 1. At z similar to 1 many galaxies with large internal color dispersion are spirals, with a few irregular and interacting systems. Few z similar to 2.3 galaxies have high internal color dispersion, and we infer that those that do are also actively merging. Using simple models, we interpret the change in the total color and internal color dispersion as evidence for the presence of older and more diverse stellar populations at z similar to 1 that are not generally present at z greater than or similar to 2. The z similar to 2.3 galaxies do not increase their stellar diversity as rapidly as they could given basic timescale arguments and simple models. We conclude that the star formation histories of galaxies at z greater than or similar to 2 are dominated by discrete, recurrent starbursts, which quickly homogenize the galaxies' stellar content and are possibly associated with mergers. The increase in the diversification of stellar populations by z similar to 1 implies that merger-induced starbursts occur less frequently than at higher redshifts, and more quiescent modes of star formation become the dominant mechanism. This transition in the mode of star formation coincides with the emergence of Hubble-sequence galaxies, which seems to occur around z similar to 1.4.