The dependence of star formation activity on environment and stellar mass at z ∼ 1 from the HiZELS-Hα survey?

This paper presents an environment and stellar mass study of a large sample of star-forming Hα emitters at z = 0.84 from the High-z Emission Line Survey (HiZELS), over 1.3 deg2 split over two fields (COSMOS and UKIDSS UDS). By taking advantage of a truly panoramic coverage of a wide range of environments, from the field to a rich cluster, it is shown that both stellar mass and environment play crucial roles in determining the properties of star-forming galaxies. Specific star formation rates (sSFRs) decline with stellar mass in all environments, and the fraction of Hα star-forming galaxies declines sharply from ≈ 40 per cent for galaxies with masses around 1010 M to effectively zero above 1011.5 M, confirm-ing that mass-downsizing is generally in place by z ∼ 1. The fraction of star-forming galaxies is also found to fall sharply as a function of local environmental density from ≈ 40 per cent in the field to approaching zero at rich group/cluster densities. When star formation does occur in such high density regions, it is found to be mostly dominated by potential mergers and, indeed, if only non-merging star-forming galaxies are considered, then the environment and mass trends are even stronger and are qualitatively similar at all masses and environments, re