The Evolution of the Star Formation Activity in Galaxies and Its Dependence on Environment

We study how the proportion of star-forming galaxies evolves between z = 0.8 and z = 0 as a function of galaxy environment, using the [OII] line in emission as a signature of ongoing star formation. Our high-z dataset comprises 16 clusters, 10 groups and another 250 galaxies in poorer groups and the field at z = 0.4 − 0.8 from the ESO Distant Cluster Survey, plus another 9 massive clusters at similar redshifts. As a local comparison, we use samples of galaxy systems selected from the Sloan Digital Sky Survey at 0.04 < z < 0.08. At highz most systems follow a broad anticorrelation between the fraction of star-forming galaxies and the system velocity dispersion. At face value, this suggests that at z = 0.4 − 0.8 the mass of the system largely determines the proportion of galaxies with ongoing star formation. At these redshifts the strength of star formation (as measured by the [OII] equivalent width) in star-forming galaxies is also found to vary systematically with environment. Sloan clusters have much lower fractions of star-forming galaxies than clusters at z = 0.4 − 0.8 and, in contrast with the distant clusters, show a plateau for velocity dispersions ≥ 550kms−1, where the fraction of galaxies with [OII] emission does not vary systematically with velocity dispersion. We quantify the evolution of the proportion of star-forming galaxies as a function of the system velocity dispersion and find it is strongest in intermediate-mass systems (σ ∼ 500 − 600kms−1 at z=0). To understand the origin of the observed trends, we use the Press-Schechter formalism and the Millennium Simulation and show that galaxy star formation histories may be closely related to the growth history of clusters and groups. We consider a scenario in which the population of passive galaxies (those devoid of ongoing star formation at the time they are observed) consists of two components: “primordial” passive galaxies whose stars all formed at z > 2.5 and “quenched” galaxies whose star formation has been truncated due to the dense environment at later times. We propose a scheme that is able to account for the observed relations between the star-forming fraction and σ in clusters at highand low-z. If this scenario is roughly correct, the link between galaxy properties and environment is extremely simple to predict purely from a knowledge of the growth of dark matter structures. Subject headings: galaxies: clusters: general — galaxies: evolution — galaxies: stellar content — galaxies: fundamental parameters — cosmology: observations