The mass assembly history of field galaxies: Detection of an evolving mass limit for starforming galaxies

We characterize the mass-dependent evolution of more than 8000 galaxies using spectroscopic redshifts from the DEEP2 Galaxy Redshift Survey in the range 0: 4 < z < 1: 4 and stellar masses calculated from K-band photometry obtained at Palomar Observatory. This sample spans more than 1.5 deg(2) in four independent fields. Using rest-frame U-B color and [O II] equivalent widths, we distinguish star-forming from passive populations in order to explore the nature of "downsizing'' - a pattern in which the sites of active star formation shift from high-mass galaxies at early times to lower mass systems at later epochs. We identify a mass limit, M-Q, above which star formation appears to be quenched and show that the physical mechanisms responsible for downsizing can thus be empirically quantified by charting the evolution in this threshold mass. We find that MQ decreases with time by a factor of similar to 3 across our redshift range according to M-Q proportional to (1 + z)(3.5). To further constrain possible quenching mechanisms, we investigate how downsizing depends on local galaxy environment using the projected third-nearest-neighbor statistic D-p,(3). For the majority of galaxies near the median density, there is no significant correlation between downsizing and environment. However, a trend is observed in the comparison between environments that are more than 3 times overdense or underdense relative to the median. Here, downsizing appears accelerated in overdense regions that host higher numbers of massive, early-type galaxies as compared to the underdense regions. Our results significantly constrain recent suggestions for the origin of downsizing and indicate that the process for quenching star formation must, primarily, be internally driven.