From fields to a super-cluster: the role of the environment at z=0.84 with HiZELS

TL;DR

This study uses a large H-alpha survey at z=0.84 to show that galaxy star formation activity decreases with environmental density, with mass and environment jointly influencing galaxy evolution.

Contribution

It provides new insights into the environmental dependence of galaxy star formation at z~1 using the widest and deepest H-alpha survey at this redshift.

Findings

Star-forming galaxy fraction decreases from 40% in fields to near 0% in clusters.

Median SFR increases with density for low and medium mass galaxies.

Environment influences the faint-end slope of the H-alpha luminosity function.

Abstract

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 ~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~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~1: stellar mass is the primary predictor of star formation activity, but the environment also plays a major role.