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 timepartitioned according to galaxy morphology. Using a well-defined catalog of2150 galaxies based, in part, on archival data in the GOODS fields, we assignmorphological types in three broad classes (Ellipticals, Spirals,Peculiar/Irregulars) to a limit of z_AB=22.5 and make the resulting catalogpublicly available. We combine redshift information, optical photometry fromthe GOODS catalog and deep K-band imaging to assign stellar masses. We findlittle evolution in the form of the galaxy stellar mass function from z~1 toz=0, especially at the high mass end where our results are most robust.Although the population of massive galaxies is relatively well established atz~1, its morphological mix continues to change, with an increasing proportionof early-type galaxies at later times. By constructing type-dependent stellarmass functions, we show that in each of three redshift intervals, E/S0'sdominate the higher mass population, while spirals are favored at lower masses.This transition occurs at a stellar mass of 2--3 times 10^{10} Msun at z~0.3(similar to local studies) but there is evidence that the relevant mass scalemoves to higher mass at earlier epochs. Such evolution may represent themorphological extension of the ``downsizing'' phenomenon, in which the mostmassive galaxies stop forming stars first, with lower mass galaxies becomingquiescent later. We infer that more massive galaxies evolve into spheroidalsystems at earlier times, and that this morphological transformation may onlybe completed 1--2 Gyr after the galaxies emerge from their active star formingphase. We discuss several lines of evidence suggesting that merging may play akey role in generating this pattern of evolution.Comment: 24 pages, 1 table, 8 figures, accepted for publication in Ap