HiZELS: a high-redshift survey of Hα emitters - II. The nature of star-forming galaxies at z = 0.84
Ivison, R. J.; Smail, Ian; Cirasuolo, M.; Coppin, K.; Geach, J. E.; Kurk, J.; Sobral, D.; Best, P. N.; Casali, M.; Dalton, G. B.
United Kingdom, Germany
Abstract
New results from a large survey of Hα 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 FHα ~ 10-16ergs-1cm-2 in a comoving volume of 1.8 × 105Mpc3, resulting in the largest and deepest survey of its kind ever done at z ~ 1. There are 1517 potential line emitters detected across ~1.4 deg2, of which 743 are selected as Hα emitters, based on their photometric and spectroscopic redshifts. These are then used to calculate the Hα luminosity function, which is well fitted by a Schechter function with L* = 1042.26+/-0.05ergs-1, φ* = 10-1.92+/-0.10Mpc-3 and α = -1.65 +/- 0.15, and are used to estimate the volume average star formation rate (SFR) at z = 0.845, ρSFR: 0.15 +/- 0.01Msolaryr-1Mpc-3 (corrected for 15 per cent active galactic nucleus contamination and integrated down to 2.5Msolaryr-1). These results robustly confirm a strong evolution of ρSFR from the present day out to z ~ 1 and then flattening to z ~ 2 using a single star formation indicator: Hα luminosity. Out to z ~ 1, both the characteristic luminosity and space density of the Hα emitters increase significantly; at higher redshifts, L* continues to increase, but φ* decreases. The z = 0.84 Hα 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 ~20 per cent of the total integrated ρSFR at this redshift. Irregulars and mergers dominate the Hα 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 ~ 1, although the continued strong evolution of L* beyond z = 1 suggests an increasing importance of merger activity at higher redshifts.
Based on observations obtained with the Wide Field CAMera (WFCAM) on the United Kingdom Infrared Telescope (UKIRT). E-mail: drss@roe.ac.uk