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 0 as a function of galaxy environment, using the O II line in emission as a signature of ongoing star formation. Our high-z data set 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 galaxy systems selected from the Sloan Digital Sky Survey (SDSS) at 0.04 < z < 0.08. At high z 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 O II equivalent width) in star-forming galaxies is also found to vary systematically with environment. SDSS clusters have much lower fractions of starforming galaxies than clusters at z = 0.4-0.8 and, in contrast with the distant clusters, show a plateau for velocity dispersions ≥550 km s^-1, where the fraction of galaxies with O II 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 that it is strongest in intermediate-mass systems (σ ∼ 500-600 km s^-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. If the scenario we propose 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.