Integral Field Spectroscopy of High-Redshift Star-forming Galaxies with Laser-guided Adaptive Optics: Evidence for Dispersion-dominated Kinematics

We present early results from an ongoing study of the kinematic structure of star-forming galaxies at redshift z~2-3 using integral-field spectroscopy of rest-frame optical nebular emission lines in combination with Keck laser guide star adaptive optics (LGSAO). We show kinematic maps of three target galaxies Q1623-BX453, Q0449-BX93, and DSF 2237a-C2 located at redshifts z=2.1820, 2.0067, and 3.3172, respectively, each of which is well resolved with a PSF measuring approximately 0.11"-0.15" (~900-1200 pc at z~2-3) after cosmetic smoothing. Neither galaxy at z~2 exhibits substantial kinematic structure on scales >~30 km s^-1 both are instead consistent with largely dispersion-dominated velocity fields with σ~80 km s^-1 along any given line of sight into the galaxy. In contrast, DSF 2237a-C2 presents a well-resolved gradient in velocity over a distance of ~4 kpc with peak-to-peak amplitude of 140 km s^-1. It is unlikely that DSF 2237a-C2 represents a dynamically cold rotating disk of ionized gas as the local velocity dispersion of the galaxy (σ=79 km s^-1) is comparable to the observed shear. While some gas cooling models reproduce the observed kinematics better than a simple rotating disk model, even these provide a poor overall description of the target galaxies, suggesting that our current understanding of gas cooling mechanisms in galaxies in the early universe is (at best) incomplete.

Based 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.