The dependence of star formation activity on environment and stellar
  mass at z~1 from the HiZELS H-alpha survey
  (Abridged) This paper presents an environment and stellar mass study of a
large sample of star-forming (SF) galaxies at z=0.84 from the HiZELS survey,
over 1.3 deg^2 in the COSMOS and UKIDSS UDS fields. By taking advantage of a
truly panoramic coverage, from the field to a rich cluster, it is shown that
both mass and environment play crucial roles in determining the properties of
SF galaxies. The median specific SFR declines with mass in all environments,
and the fraction of galaxies forming stars declines from ~40%, for M~10^10M_sun
to effectively zero at M>10^11.5M_sun, confirming that mass-downsizing is
generally in place by z~1. The fraction of SF galaxies also falls as a function
of local environmental density from ~40% in the field to approaching zero at
rich group/cluster densities. When SF does occur in high density regions, it is
merger-dominated and, if only non-merging SF galaxies are considered, then the
environment and mass trends are even stronger and largely independent, as in
the local Universe. The median SFR of SF galaxies is found to increase with
density up to intermediate (group or cluster outskirts) densities; this is
clearly seen as a change in the faint-end slope of the H-alpha LF from steep
(-1.9), in poor fields, to shallow (-1.1) in groups and clusters.
Interestingly, the relation between median SFR and environment is only found
for low to moderate-mass galaxies (below ~10^10.6M_sun), and is not seen for
massive SF galaxies. Overall, these observations provide a detailed view over a
sufficiently large range of mass and environment to reconcile previous
observational claims: mass is the primary predictor of SF activity at z~1, but
the environment, while enhancing the median SFR of (lower-mass) SF galaxies, is
ultimately responsible for suppressing SF activity in all galaxies above
surface densities of 10-30 Mpc^-2 (groups and clusters).
