HiZELS: a high-redshift survey of Hα emitters - II. The nature of star-forming galaxies at z=0.84☆

New results from a large survey of H alpha emission-line galaxies at z = 0.84 using the Wide Field Camera on the United Kingdom Infrared Telescope and a custom narrow-band filter in the J band are presented as part of the High-z Emission Line Survey (HiZELS). The deep narrow-band images reach an effective flux limit of F-H alpha similar to 10(-16) erg s(-1) cm(-2) in a comoving volume of 1.8 x 10(5) Mpc(3), resulting in the largest and deepest survey of its kind ever done at z similar to 1. There are 1517 potential line emitters detected across similar to 1.4 deg(2), of which 743 are selected as H alpha emitters, based on their photometric and spectroscopic redshifts. These are then used to calculate the H alpha luminosity function, which is well fitted by a Schechter function with L* = 10(42.26 +/- 0.05) erg s(-1), * = 10(-1.92 +/- 0.10) Mpc(-3) and alpha = -1.65 +/- 0.15, and are used to estimate the volume average star formation rate (SFR) at z = 0.845, (SFR): 0.15 +/- 0.01 M-circle dot yr(-1) Mpc(-3) (corrected for 15 per cent active galactic nucleus contamination and integrated down to 2.5 M-circle dot yr(-1)). These results robustly confirm a strong evolution of (SFR) from the present day out to z similar to 1 and then flattening to z similar to 2 using a single star formation indicator: H alpha luminosity. Out to z similar to 1, both the characteristic luminosity and space density of the H alpha emitters increase significantly; at higher redshifts, L* continues to increase, but * decreases. The z = 0.84 H alpha emitters are mostly disc galaxies (82 +/- 3 per cent), while 28 +/- 4 per cent of the sample shows signs of merger activity; mergers account for similar to 20 per cent of the total integrated (SFR) at this redshift. Irregulars and mergers dominate the H alpha luminosity function above L*, while discs are dominant at fainter luminosities. These results demonstrate that it is the evolution of 'normal' disc galaxies that drives the strong increase in the SFR density from the current epoch to z similar to 1, although the continued strong evolution of L* beyond z = 1 suggests an increasing importance of merger activity at higher redshifts.