Obscured star formation at z=0.84 with HiZELS☆: the relationship between star formation rate and Hα or ultraviolet dust extinction

We compare H alpha, ultraviolet (UV) and infrared (IR) indicators of star formation rate (SFR) for a well-defined sample of z = 0.84 emission-line galaxies from the High-z Emission Line Survey (HiZELS). Using emission-line, optical, IR, radio and X-ray diagnostics, we estimate that 5-11 per cent of H alpha emitters at this redshift are active galactic nuclei. We detect 35 per cent of the H alpha emitters individually at 24 mu m, and stack the locations of star-forming emitters on deep 24-mu m Spitzer Space Telescope images in order to calculate the typical SFRs of our H alpha-emitting galaxies. These are compared to the observed H alpha line fluxes in order to estimate the extinction at z = 0.84, and we find a significant increase in dust extinction for galaxies with higher SFRs. We demonstrate that the relationship between SFR and extinction found in the local Universe is also suitable for our high-redshift galaxies, and attribute the overall increase in the typical dust extinction for z = 0.84 galaxies to an increase in the average SFR, rather than to a change in dust properties at higher redshift. We calculate the UV extinction, and find a similar dependence on SFR to the H alpha results, but no evidence for a 2175 A UV bump in the dust attenuation law for high-redshift star-forming galaxies. By comparing H alpha and UV indicators, we calculate the conversion between the dust attenuation of nebular and stellar radiation, gamma, and show that gamma = 0.50 +/- 0.14. The extinction/SFR relationship is shown to be applicable to galaxies with a range of morphologies and bulge-to-disc ratios, to both merging and non-merging galaxies, and to galaxies within high- and low-density environments, implying that it is a fundamental property of star-forming regions. In order to allow future studies to easily correct for a SFR-dependent amount of dust extinction, we present an equation to predict the extinction of a galaxy, based solely on its observed H alpha luminosity, and use this to recalculate the H alpha luminosity function and SFR density at z = 0.84.