Hα observations of a large sample of galaxies at z ∼ 2:: Implications for star formation in high-redshift galaxies

Using H alpha spectra of 114 rest-frame UV-selected galaxies at z similar to 2, we compare inferred star formation rates (SFRs) with those determined from the UV continuum luminosity. After correcting for extinction using standard techniques based on the UV continuum slope, we find excellent agreement between the indicators, with < SFRH alpha > = 31 M-circle dot yr(-1) and < SFRUV > 29 M-circle dot yr(-1). The agreement between the indicators suggests that the UV luminosity is attenuated by a typical factor of similar to 4.5 (ranging from no attenuation to a factor of similar to 100 for the most obscured object in the sample), in good agreement with estimates of obscuration from X-ray, radio, and mid-IR data. The H alpha luminosity is attenuated by a factor of similar to 1.7 on average, and the maximum H alpha attenuation is a factor of similar to 5. In agreement with X-ray and mid-IR studies, we find that the SFR increases with increasing stellar mass and at brighter K magnitudes to < SFRH alpha > 60 M-circle dot yr(-1) for galaxies with K-s < 20; the correlation between K magnitude and SFR is much stronger than the correlation between stellar mass and SFR. All galaxies in the sample have SFRs per unit area Sigma SFR in the range observed in local starbursts. We compare the instantaneous SFRs and the past average SFRs as inferred from the ages and stellar masses, finding that for most of the sample, the current SFR is an adequate representation of the past average. There is some evidence that the most massive galaxies (M-star > 10(11) M circle dot) have had higher SFRs in the past.