Swirls of FIRE: spatially resolved gas velocity dispersions and star formation rates in FIRE-2 disc environments

Abstract

We study the spatially resolved (sub-kpc) gas velocity dispersion (sigma)-star formation rate (SFR) relation in the FIRE-2 (Feedback in Realistic Environments) cosmological simulations. We specifically focus on Milky Way-mass disc galaxies at late times (z approximate to 0). In agreement with observations, we find a relatively flat relationship, with sigma approximate to N 15-30 km s(-1) in neutral gas across 3 dex in SFRs. We show that higher dense gas fractions (ratios of dense gas to neutral gas) and SFRs arc correlated at constant sigma. Similarly, lower gas fractions (ratios of gas to stellar mass) are correlated with higher sigma at constant SFR. The limits of the sigma-Sigma(SFR) relation correspond to the onset of strong outflows. We see evidence of 'on-off' cycles of star formation in the simulations, corresponding to feedback injection time-scales of 10-100 Myr, where SFRs oscillate about equilibrium SFR predictions. Finally, SFRs and velocity dispersions in the simulations agree well with feedback-regulated and marginally stable gas disc (Toomre's Q = 1) model predictions, and the simulation data effectively rule out models assuming that gas turns into stars at (low) constant efficiency (i.e. 1 per cent per free-fall time). And although the simulation data do not entirely exclude gas accretion/gravitationally powered turbulence as a driver of sigma, it appears to be subdominant to stellar feedback in the simulated galaxy discs at z approximate to 0. Show more