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.