From Fields to a Super-Cluster: The Role of the Environment at z=0.84 with HiZELS

At z = 0, clusters are primarily populated by red, elliptical and massive galaxies, while blue, spiral and lower-mass galaxies are common in low-density environments. Understanding how and when these differences were established is of absolute importance for our understanding of galaxy formation and evolution, but results at high-z remain contradictory. By taking advantage of the widest and deepest H alpha narrow-band survey at z = 0.84 over the COSMOS and UKIDSS UDS fields, probing a wide range of densities (from poor fields to rich groups and clusters, including a confirmed super-cluster with a striking filamentary structure), we show that the fraction of star-forming galaxies falls continuously from similar to 40% in fields to approaching 0% in rich groups/clusters. We also find that the median SFR increases with environmental density, at least up to group densities - but only for low and medium mass galaxies, and thus such enhancement is mass-dependent at z similar to 1. The environment also plays a role in setting the faint-end slope (alpha) of the H alpha luminosity function. Our findings provide a sharper view on galaxy formation and evolution and reconcile previously contradictory results at z similar to 1: stellar mass is the primary predictor of star formation activity, but the environment also plays a major role.