This paper presents an environment and stellar mass study of a large sample of star-forming H \alpha emitters at z = 0.84 from the High- z Emission Line Survey ( HiZELS ) , over 1.3 deg ^ { 2 } 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 H \alpha star-forming galaxies declines sharply from \approx 40 per cent for galaxies with masses around 10 ^ { 10 } M _ { \odot } to effectively zero above 10 ^ { 11.5 } M _ { \odot } , confirming that mass-downsizing is generally in place by z \sim 1 .
The fraction of star-forming galaxies is also found to fall sharply as a function of local environmental density from \approx 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 , respectively , as in the local Universe .
The median star-formation rate of H \alpha emitters at z = 0.84 is found to increase with density for both field and intermediate ( group or cluster outskirts ) densities ; this is clearly seen as a change in the faint-end slope of the H \alpha luminosity function from steep ( \alpha \approx - 1.9 ) , in poor fields , to shallow ( \alpha \approx - 1.1 ) in groups and clusters .
Interestingly , the relation between median SFR and environment is only found for low to moderate-mass galaxies ( with stellar masses below about 10 ^ { 10.6 } M _ { \odot } ) , and is not seen for the most massive star-forming galaxies .
Overall , these observations provide a detailed view over a sufficiently large range of mass and environment to reconcile previous observational claims : stellar mass is the primary predictor of star-formation activity at z \sim 1 , but the environment , while initially enhancing the median SFR of ( lower-mass ) star-forming galaxies , is ultimately responsible for suppressing star-formation activity in all galaxies above surface densities of 10-30 Mpc ^ { -2 } ( group and cluster environments ) .