What drives the kinematic evolution of star-forming galaxies?

One important result from recent large integral field spectrograph (IFS) surveys is that the intrinsic velocity dispersion of galaxies increases with redshift. Massive, rotationdominated discs are already in place at z ~ 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. These simulations reproduce the observed trends between intrinsic velocity dispersion (σ intr), SFR, and z. In both the observed and simulated galaxies, σ intr is positively correlated with SFR. σ intr increases with redshift out to z ~ 1 and then flattens beyond that. In the FIRE simulations, σ intr can vary significantly on timescales of 100 Myr. These variations closely mirror the time evolution of the SFR and gas inflow rate ( gas). By cross-correlating pairs of σ intr gas, and SFR, we show that the increased gas inflow leads to subsequent enhanced star formation, and enhancements in σ intr tend to temporally coincide with increases in gas and SFR.