Measurement of Star-Formation Rate from H-alpha in field galaxies at z=1

  • 25 August 1998
Abstract
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 corresponding to a luminosity limit of \~10^{41} ergs at this redshift for a H_0=50 km/s/Mpc 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 (2800 Angstroms) luminosity. The mean star-formation rate at z=1, from this sample, is found to be three times as high as the ultraviolet estimates. The dust extinction in these galaxies is inferred to be moderate, with a typical A_V=0.5-1.0 mags, comparable to local field galaxies. This suggests that the bulk of star-formation is not heavily obscured. 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. The star-formation history of the Universe from Balmer lines is compiled and found to be 2-3x higher than that inferred from the UV at all redshifts. It can not yet be clearly established whether the star-formation rate falls off or remains constant at high-redshift.

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