Swirls of FIRE: Spatially Resolved Gas Velocity Dispersions and Star
  Formation Rates in FIRE-2 Disk Environments
  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 disk galaxies at late times. In agreement with observations, we
find a relatively flat relationship, with $\sigma \approx 15-30$ 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
$\sigma$. Similarly, lower gas fractions (ratios of gas to stellar mass) are
correlated with higher $\sigma$ at constant SFR. The limits of the
$\sigma$-$\Sigma_{\rm SFR}$ 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., ${\rm 1\%}$ 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 strongly subdominant to
stellar feedback in the simulated galaxy disks.
