The Evolution and Origin of Ionized Gas Velocity Dispersion from z ∼ 2.6 to z ∼ 0.6 with KMOS ^3D

We present the 0.6 < z < 2.6 evolution of the ionized gas velocity dispersion in 175 star-forming disk galaxies based on data from the full integral field spectroscopic survey. In a forward-modeling Bayesian framework including instrumental effects and beam-smearing, we fit simultaneously the observed galaxy velocity and velocity dispersion along the kinematic major axis to derive the intrinsic velocity dispersion σ ₀. We find a reduction of the average intrinsic velocity dispersion of disk galaxies as a function of cosmic time, from σ ₀ ∼ 45 km s ^-1 at z ∼ 2.3 to σ ₀ ∼ 30 km s ^-1 at z ∼ 0.9. There is substantial intrinsic scatter (σ _σ0,int ≈ 10km s ^-1) around the best-fit σ ₀-z relation beyond what can be accounted for from the typical measurement uncertainties (δσ ₀ ≈ 12 km s ^-1), independent of other identifiable galaxy parameters. This potentially suggests a dynamic mechanism such as minor mergers or variation in accretion being responsible for the scatter. Putting our data into the broader literature context, we find that ionized and atomic+molecular velocity dispersions evolve similarly with redshift, with the ionized gas dispersion being ∼10-15 km s ^-1 higher on average. We investigate the physical driver of the on average elevated velocity dispersions at higher redshift and find that our galaxies are at most marginally Toomre-stable, suggesting that their turbulent velocities are powered by gravitational instabilities, while stellar feedback as a driver alone is insufficient. This picture is supported through comparison with a state-of-the-art analytical model of galaxy evolution.