Measurement of the star formation rate from Hα in field galaxies at z=1

We report the results of J-band infrared spectroscopy of a sample of 13 z=1 field galaxies drawn from the Canada-France Redshift Survey, targeting galaxies whose redshifts place the rest frame H-alpha line emission from HII regions in between the bright night sky OH lines. As a result we detect emission down to a flux limit of ~10^{-16} ergs cm^{-2} s^{-1} corresponding to a luminosity limit of ~10^{41} ergs at this redshift for a H_0=50 km s^{-1} Mpc,^{-1} q_0=0.5 cosmology. From these luminosities we derive estimates of the star-formation rates in these galaxies which are independent of previous estimates based upon their rest-frame ultraviolet (2800A) luminosity. The mean star-formation rate at z=1, from this sample, is found to be at least three times as high as the ultraviolet estimates. The standard dust extinction in these galaxies is inferred to be A_V=0.5-1.0 mags, comparable to local field galaxies, suggesting that the bulk of star-formation is not heavily obscured unless one uses greyer extinction laws. Star-forming galaxies have the bluest colours and a preponderance of disturbed/interacting morphologies. We also investigate the effects of particular star-formation histories, in particular the role of bursts vs continuous star-formation in changing the detailed distribution of UV to H-alpha emission. Generally we find that models dominated by short, overlapping, bursts at typically 0.2 Gyr intervals provide a better model for the data than a constant rate of star-formation. The star-formation history of the Universe from Balmer lines is compiled and found to be typically 2--3\times higher than that inferred from the UV {\em at all redshifts}. It can not yet be clearly established whether the star-formation rate falls off or remains constant at high-redshift.