What drives the evolution of gas kinematics in star-forming galaxies?

One important result from recent large integral field spectrograph (IFS) surveys is that the intrinsic velocity dispersion of galaxies traced by star-forming gas increases with redshift. Massive, rotation-dominated discs are already in place at z similar to 2, but they are dynamically hotter than spiral galaxies in the local Universe. Although several plausible mechanisms for this elevated velocity dispersion (e.g. star formation feedback, elevated gas supply, or more frequent galaxy interactions) have been proposed, the fundamental driver of the velocity dispersion enhancement at high redshift remains unclear. We investigate the origin of this kinematic evolution using a suite of cosmological simulations from the FIRE (Feedback In Realistic Environments) project. Although IFS surveys generally cover a wider range of stellar masses than in these simulations, the simulated galaxies show trends between intrinsic velocity dispersion (sigma(intr)), SFR, and z in agreement with observations. In both observations and simulations, galaxies on the star-forming main sequence have median sintr values that increase from z similar to 0 to z similar to 1-1.5, but this increasing trend is less evident at higher redshift. In the FIRE simulations, sintr can vary significantly on time-scales of less than or similar to 100 Myr. These variations closely mirror the time evolution of the SFR and gas inflow rate ((M) over dot(gas)). By cross-correlating pairs of sigma(intr), (M) over dot(gas), and SFR, we show that increased gas inflow leads to subsequent enhanced star formation, and enhancements in sigma(intr) tend to temporally coincide with increases in. (M) over dot(gas) and SFR.