The Assembly of Diversity in the Morphologies and Stellar Populations of High–redshift Galaxies1

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 ∼ 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 ∼ 2.3 all have compact and irregular rest–frame optical morphologies with little difference between their rest–frame UV–optical morphologies, and with no clearly evident Hubble–sequence candidates. The mean galaxy size increases from z ∼ 2.3 to 1 by roughly 40%, and the number density of galaxies larger than 3 kpc increases by a factor of ≈ 7. The size–luminosity distribution of z ∼ 1 galaxies is broadly consistent with that in the local universe, allowing for passive evolution. However, we argue that galaxies at z ∼ 2.3 are not the fully formed progenitors of present–day galaxies, and they must continue to grow in both their sizes and stellar masses. We have quantified the differences in morphology 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 ∼ 2.3 to 1. At z ∼ 1 many galaxies with large internal color dispersion are spirals, with a few irregular and interacting systems. Few z ∼ 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 ∼ 1 that are not generally present at z & 2. The z ∼ 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 & 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 ∼ 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 ∼ 1.4. Subject headings: cosmology: observations — galaxies: evolution — galaxies: formation — galaxies: highredshift — galaxies: stellar content — galaxies: structure