What drives the kinematic evolution of star-forming galaxies?

Abstract

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