z ∼ 2: An Epoch of Disk Assembly

Abstract

We explore the evolution of the internal gas kinematics of star-forming galaxies from the peak of cosmic star formation at z∼ 2 to today. Measurements of galaxy rotation velocity V _rot, which quantify ordered motions, and gas velocity dispersion {σ }_g, which quantify disordered motions, are adopted from the DEEP2 and SIGMA surveys. This sample covers a continuous baseline in redshift over 0.1 < z < 2.5, spanning 10 Gyr. At low redshift, nearly all sufficiently massive star-forming galaxies are rotationally supported ({V}_{rot}> {σ }_g). By z = 2, 50% and 70% of galaxies are rotationally supported at low ({10}⁹{--}{10}¹⁰ {M}_⊙ ) and high ({10}¹⁰{--}{10}¹¹ {M}_⊙ ) stellar mass, respectively. For {V}_{rot} > 3 {σ }_g, the percentage drops below 35% for all masses. From z = 2 to now, galaxies exhibit remarkably smooth kinematic evolution on average. All galaxies tend toward rotational support with time, and higher-mass systems reach it earlier. This is largely due to a mass-independent decline in {σ }_g by a factor of 3 since z = 2. Over the same time period, V _rot increases by a factor of 1.5 in low-mass systems but does not evolve at high mass. These trends in V _rot and {σ }_g are at a fixed stellar mass and therefore should not be interpreted as evolutionary tracks for galaxy populations. When populations are linked in time via abundance matching, {σ }_g declines as before and V _rot strongly increases with time for all galaxy populations, enhancing the evolution in {V}_{rot}/{σ }_g. These results indicate that z=2 is a period of disk assembly, during which strong rotational support is only just beginning to emerge.