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~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, SFR, and redshift in agreement with observations. In both
the observed and simulated galaxies, intrinsic velocity dispersion is
positively correlated with SFR. Intrinsic velocity dispersion increases with
redshift out to z~1 and then flattens beyond that. In the FIRE simulations,
intrinsic velocity dispersion can vary significantly on timescales of <100 Myr.
These variations closely mirror the time evolution of the SFR and gas inflow
rate. By cross-correlating pairs of intrinsic velocity dispersion, gas inflow
rate, and SFR, we show that increased gas inflow leads to subsequent enhanced
star formation, and enhancements in intrinsic velocity dispersion tend to
temporally coincide with increases in gas inflow rate and SFR.
