The Relation between Star Formation, Morphology and Local Density in High Redshift Clusters and Groups

We investigate how the [OII] properties and the morphologies of galaxies in clusters and groups at z = 0.4− 0.8 depend on projected local galaxy density. We also compare our results with those derived from the field at similar redshifts and from clusters at low-z. In both nearby and distant clusters, higher-density regions contain proportionally fewer star-forming galaxies. At odds with low-z results, the relation between star-forming fraction and local density at high-z varies from highto low-mass clusters, although is not solely a function of cluster mass. Both at highand low-z, the average [OII] equivalent width (EW) of star-forming galaxies is independent of local density. However, in distant clusters the average current star formation rate in star-forming galaxies seems to peak at densities ∼ 15− 40 galaxies Mpc−2. When using galaxy samples with similar mass distributions, we do not find large variations in the average EW(OII) and SFR properties of star-forming galaxies in the distant field, poor groups, and clusters. Overall, these results suggest that at high-z the current star formation activity in star-forming galaxies of a given galaxy mass does not depend strongly on global or local environment, though the possible SFR peak at intermediate densities seems at odds with this conclusion. We derive cluster-integrated star formation rates and find that the cluster SFR normalized by cluster mass anticorrelates with mass and correlates with the star-forming fraction, although we caution that the anticorrelation with mass could be mainly driven by correlated errors. We demonstrate that these trends can be understood given a) that the average star-forming galaxy forms about one solar mass per year (uncorrected for dust) in all our clusters; b) how the total number of galaxies scales with cluster mass; and c) the dependence of star-forming fraction on cluster mass. These findings show that the most fundamental open question currently remains why the evolution of the star-forming fraction depends on cluster mass. We present the morphologydensity relation for our z = 0.4−0.8 clusters, and uncover that the decline of the spiral fraction with density is entirely driven by galaxies of types Sc or later, while early spirals (Sa’s and Sb’s) have a flat distribution with density, similarly to that of S0s. For galaxies of a given Hubble type, we see no evidence that star formation properties (ongoing SFR, [OII] equivalent width, 4000 Å break) depend on local environment. In contrast with recent findings at low-z, in our distant clusters the star formation-density relation and the morphology-density relation are equivalent, suggesting that neither of the two relations is more fundamental than the other. We suggest that the contrasting results at highand low-z are partly due to different classification methods and/or to evolutionary effects giving rise to a progressive decoupling of the two relations at recent epochs. Subject headings: galaxies: clusters: general — galaxies: evolution — galaxies: stellar content