H-alpha Spectroscopy of Galaxies at z>2: Kinematics and Star Formation

Abstract

We present near-infrared spectroscopy of H-alpha emission lines in a sample of 16 star-forming galaxies at redshifts 2.0<z<2.6. Our targets are drawn from a large sample of galaxies photometrically selected and spectroscopically confirmed to lie in this redshift range. Six of the galaxies exhibit spatially extended, tilted H-alpha emission lines; rotation curves for these objects reach mean velocities of ~150 km/s at radii of ~6 kpc, without corrections for any observational effects. The velocities and radii give a mean dynamical mass of M>4e10 M_sun. One-dimensional velocity dispersions for the 16 galaxies range from ~50 to ~260 km/s, and in cases where 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 to 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 find a mean SFR_H-alpha of 16 M_sun/yr and an average SFR_H-alpha/SFR_UV ratio of 2.4, without correcting for extinction. We see moderate evidence for an inverse correlation between the UV continuum luminosity and the ratio SFR_H-alpha/SFR_UV, 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.

20 pages, 9 figures. Accepted by ApJ