Star formation and AGN activity in SDSS cluster galaxies

We investigate the recent and current star formation activity of galaxies as function of distance from the cluster centre in a sample of 521 Sloan Digital Sky Survey clusters at z < 0.1. We characterize the recent star formation history (SFH) by the strength of the 4000 A break and the strength of the Balmer absorption lines, and thus probe the SFH over the last similar to 2 Gyr. We show that when the brightest cluster galaxies are excluded from the galaxy sample, there is no evidence for mass segregation in the clusters, so that differences in cluster and field populations cannot simply be attributed to different mass functions. We find a marked star formation-radius relation in that almost all galaxies in the cluster core are quiescent, i.e. have terminated star formation a few Gyr ago. This star formation-radius relation is most pronounced for low-mass galaxies and is very weak or absent beyond the virial radius. The typical star formation rate (SFR) of non-quiescent galaxies declines by approximately a factor of 2 towards the cluster centre. However, the fraction of galaxies with young stellar populations indicating a recently completed starburst or a truncation of star formation does not vary significantly with radius. These results favour a scenario in which star formation is quenched slowly, on time-scales similar to the cluster crossing time, i.e. a few Gyr. The fraction of star-forming galaxies which host a powerful optical active galactic nucleus (AGN) is also independent of clustercentric radius, indicating that the link between star formation and AGN in these galaxies operates independent of environment. The fraction of red galaxies which host a weak optical AGN decreases, however, towards the cluster centre, with a similar time-scale as the decline of star-forming galaxies. Our results can be fully explained by a gradual decline of SFR upon infall into the cluster, and rule out significant contributions from more violent processes, at least beyond cluster radii greater than or similar to 0.1R(200).