Swirls of FIRE: Spatially Resolved Gas Velocity Dispersions and Star Formation Rates in FIRE-2 Disk Environments

Abstract

We study the spatially resolved (sub-kpc) gas velocity dispersion (σ)--star formation rate (SFR) relation in the FIRE-2 (Feedback in Realistic Environments) cosmological simulations. We specifically focus on Milky Way mass disk galaxies at late times. In agreement with observations, we find a relatively flat relationship, with σ ≈ 15−30 km/s in neutral gas across 3 dex in SFRs. We show that higher dense gas fractions (ratios of dense gas to neutral gas) and SFRs are correlated at constant σ. Similarly, lower gas fractions (ratios of gas to stellar mass) are correlated with higher σ at constant SFR. The limits of the σ-Σ_(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 timescales 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 disk (Toomre's Q=1) model predictions, and the data effectively rule out models assuming that gas turns into stars at (low) constant efficiency (i.e., 1% per free-fall time). And although the simulation data do not entirely exclude gas accretion/gravitationally powered turbulence as a driver of σ, it appears to be strongly subdominant to stellar feedback in the simulated galaxy disks.

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