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 center in a sample of 521 SDSS clusters at z < 0.1. We characterize the recent star formation history by the strength of the 4000Å break and the strength of the Balmer absorption lines, and thus probe the star formation history over the last ∼ 2Gyr. 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 lowmass galaxies and is very weak or absent beyond the virial radius. The typical star formation rate of non-quiescent galaxies declines by approximately a factor of two towards the cluster center. 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 favor a scenario in which star formation is quenched slowly, on timescales similar to the cluster crossing time, i.e. a few Gyr. The fraction of star-forming galaxies which host a powerful optical 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 center, with a similar timescale as the decline of star forming galaxies. Our results can be fully explained by a gradual decline of star formation rate upon infall into the cluster, and rule out significant contributions from more violent processes at least beyond cluster radii