Hα star formation rates in massive galaxies at z~ 1

We present a near-infrared spectroscopic study of a stellar mass selected sample of galaxies at z~ 1 utilizing the Long-slit Intermediate Resolution Infrared Spectrograph multi-object spectrograph on the William Herschel Telescope. We detect continuum, and the Hα line for our sample, which is one of the better direct tracers of star formation in external galaxies. We spectroscopically measure the Hα emission from 41 massive (M _* > 10 ^10.5M _⊙) galaxies taken from the POWIR Survey with spectroscopic redshifts 0.4 < z _spec < 1.4. We correct our Hα fluxes for dust extinction by using multiwavelength data, and investigate star formation rate (SFR) trends with mass and colour. We find a drop in the fraction of massive galaxies with M _* > 10 ¹¹M _⊙ which are detected in Hα emission at z < 0.9. We furthermore find that the fraction of galaxies with Hα emission drops steadily and significantly with redder (U-B) colours at z~ 1, and that the specific SFR (SSFR) drops with increasing (U-B) colour for galaxies at all masses. By investigating the SFR-mass relation, we find that the SFR is roughly constant with mass, in possible contrast to previous work, and that the SSFR is lower in the most massive galaxies. The scatter in the SFR versus mass relationship is very small for those systems with ongoing star formation, which suggests that star formation in the most massive galaxies at z~ 1 shuts off rather abruptly over <1Gyr, without an obvious gradual decline. We furthermore investigate the SFR as a function of (U-B) colour divided into different mass bins, revealing a tracer of the epoch of transition from star forming to passive, as a form of star formation 'downsizing'. This suggests that the shut off of star formation occurs before the change in a galaxy's colour. We find that galaxy stellar mass is the primary driving mechanism behind the star formation history for these galaxies and discuss several possible mechanisms for regulating this process.