The Deep2 Galaxy Redshift Survey: the Role of Galaxy Environment in the Cosmic Star–formation History

Using galaxy samples drawn from the Sloan Digital Sky Survey and the DEEP2 Galaxy Redshift Survey, we study the relationship between star formation and environment at z ∼ 0.1 and z ∼ 1. We estimate the total star–formation rate (SFR) and specific star–formation rate (sSFR) for each galaxy according to the measured [O II] λ3727Å nebular line luminosity, corrected using empirical calibrations to match more robust SFR indicators. Echoing previous results, we find that in the local Universe star formation depends on environment such that galaxies in regions of higher overdensity, on average, have lower star–formation rates and longer star–formation timescales than their counterparts in lower– density regions. At z ∼ 1, we show that the relationship between specific SFR and environment mirrors that found locally. However, we discover that the relationship between total SFR and overdensity at z ∼ 1 is inverted relative to the local relation. This observed evolution in the SFR–density relation is driven, in part, by a population of bright, blue galaxies in dense environments at z ∼ 1. This population, which lacks a counterpart at z ∼ 0, is thought to evolve into members of the red sequence from z ∼ 1 to z ∼ 0. Finally, we conclude that environment does not play a dominant role in the cosmic star–formation history at z < 1: the dependence of the mean galaxy SFR on local galaxy density at constant redshift is small compared to the decline in the global SFR space density over the last 7 Gyr. Subject headings: galaxies:high–redshift, galaxies:evolution, galaxies:statistics, galaxies:fundamental parameters, large–scale structure of universe