The Mass Assembly Histories of Galaxies of Various Morphologies in the GOODS Fields

We present an analysis of the growth of stellar mass with cosmic time partitioned according to galaxy morphology. Using a well-defined catalog of 2150 galaxies based, in part, on archival data in the Great Observatories Origins Deep Survey (GOODS) fields, we assign morphological types in three broad classes (Ellipticals, Spirals, Peculiar/Irregulars) to a limit of zAB=22.5 and make the resulting catalog publicly available. Utilizing 893 spectroscopic redshifts, supplemented by 1013 determined photometrically, we combine optical photometry from the GOODS catalog and deep Ks-band imaging to assign stellar masses to each galaxy in our sample. We find little evolution in the form of the galaxy stellar mass function from z ∼ 1 to z = 0, especially at the high mass end where our results are most robust. Although the population of massive galaxies is relatively well established at z ∼ 1, its morphological mix continues to change, with an increasing proportion of early-type galaxies at later times. By constructing type-dependent stellar mass functions, we show that in each of three redshift intervals, E/S0’s dominate the higher mass population, while spirals are favored at lower masses. This transition occurs at a stellar mass of 2–3×10 M⊙ at z ∼ 0.3 (similar to local studies) but there is evidence that the relevant mass scale moves to higher mass at earlier epochs. Such evolution may represent the morphological extension of the “downsizing” phenomenon, in which the most massive galaxies stop forming stars first, with lower mass galaxies becoming quiescent later. We infer that more massive galaxies evolve into spheroidal systems at earlier times, and that this morphological transformation may only be completed 1–2 Gyr after the galaxies emerge from their active star forming phase. We discuss several lines of evidence suggesting that merging may play a key role in generating this pattern of evolution. Subject headings: cosmology: observations, galaxies: formation, galaxies: evolution, morphologies 105–24 Caltech, 1201 E. California Blvd., Pasadena, CA 91125