The Evolution and Origin of Ionized Gas Velocity Dispersion from z ∼ 2.6 to z ∼ 0.6 with KMOS3D

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 KMOS^3D 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 }_{{σ 0},{int}}≈ 10 {km} {{{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.

Based on observations collected at the Very Large Telescope (VLT) of the European Southern Observatory (ESO), Paranal, Chile, under ESO program IDs 092.A-0091, 093.A-0079, 094.A-0217, 095.A-0047, 096.A-0025, 097.A-0028, 098.A-0045, 099.A-0013, 0100.A-0039, and 0101.A-0022.