Hα Spectroscopy of Galaxies at z > 2: Kinematics and Star Formation

Abstract

We present near-infrared spectroscopy of Hα emission lines in a sample of 16 star-forming galaxies at redshifts 2.0 =4×1010 Msolar. We have obtained archival Hubble Space Telescope images for two of these galaxies; they are morphologically irregular. One-dimensional velocity dispersions for the 16 galaxies range from ~50 to ~260 km s-1, and in cases in which we have both virial masses implied by the velocity dispersions and dynamical masses derived from the spatially extended emission lines, they are in rough agreement. We compare our kinematic results with similar measurements made at z~3 and find that both the observed rotational velocities and velocity dispersions tend to be larger at z~2 than at z~3. We also calculate star formation rates (SFRs) from the Hα luminosities and compare them with SFRs calculated from the UV continuum luminosity. We find a mean SFRHα of 16 Msolar yr-1 and an average SFRHα/SFRUV ratio of 2.4, without correcting for extinction. We see moderate evidence for an inverse correlation between the UV continuum luminosity and the ratio SFRHα/SFRUV, such as might be observed if the UV-faint galaxies suffered greater extinction. We discuss the effects of dust and star formation history on the SFRs and conclude that extinction is the most likely explanation for the discrepancy between the two SFRs. Based, in part, on data obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA, and was made possible by the generous financial support of the W. M. Keck Foundation. Also based in part on observations collected at the European Southern Observatory, Paranal, Chile (ESO Program 66.A-0206).