We present an analysis of the $Rlesssim 1.5$ kpc core regions of seven simulated Milky Way mass galaxies, from the FIRE-2 (Feedback in Realistic Environments) cosmological zoom-in simulation suite, for a finely sampled period ($Delta t = 2.2$ Myr) of
22 Myr at $z approx 0$, and compare them with star formation rate (SFR) and gas surface density observations of the Milky Ways Central Molecular Zone (CMZ). Despite not being tuned to reproduce the detailed structure of the CMZ, we find that four of these galaxies are consistent with CMZ observations at some point during this 22 Myr period. The galaxies presented here are not homogeneous in their central structures, roughly dividing into two morphological classes; (a) several of the galaxies have very asymmetric gas and SFR distributions, with intense (compact) starbursts occurring over a period of roughly 10 Myr, and structures on highly eccentric orbits through the CMZ, whereas (b) others have smoother gas and SFR distributions, with only slowly varying SFRs over the period analyzed. In class (a) centers, the orbital motion of gas and star-forming complexes across small apertures ($R lesssim 150$pc, analogously $|l|<1^circ$ in the CMZ observations) contributes as much to tracers of star formation/dense gas appearing in those apertures, as the internal evolution of those structures does. These asymmetric/bursty galactic centers can simultaneously match CMZ gas and SFR observations, demonstrating that time-varying star formation can explain the CMZs low star formation efficiency.
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 (Toomres $Q =1$) 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.
We explore a class of simple non-equilibrium star formation models within the framework of a feedback-regulated model of the ISM, applicable to kiloparsec-scale resolved star formation relations (e.g. Kennicutt-Schmidt). Combining a Toomre-Q-dependen
t local star formation efficiency per free-fall time with a model for delayed feedback, we are able to match the normalization and scatter of resolved star formation scaling relations. In particular, this simple model suggests that large ($sim$dex) variations in star formation rates (SFRs) on kiloparsec scales may be due to the fact that supernova feedback is not instantaneous following star formation. The scatter in SFRs at constant gas surface density in a galaxy then depends on the properties of feedback and when we observe its star-forming regions at various points throughout their collapse/star formation cycles. This has the following important observational consequences: (1) the scatter and normalization of the Kennicutt-Schmidt relation are relatively insensitive to the local (small-scale) star formation efficiency, (2) but gas depletion times and velocity dispersions are; (3) the scatter in and normalization of the Kennicutt-Schmidt relation is a sensitive probe of the feedback timescale and strength; (4) even in a model where $tilde Q_{rm gas}$ deterministically dictates star formation locally, time evolution, variation in local conditions (e.g., gas fractions and dynamical times), and variations between galaxies can destroy much of the observable correlation between SFR and $tilde Q_{rm gas}$ in resolved galaxy surveys. Additionally, this model exhibits large scatter in SFRs at low gas surface densities, in agreement with observations of flat outer HI disk velocity dispersion profiles.
Regions of disc galaxies with widespread star formation tend to be both gravitationally unstable and self-shielded against ionizing radiation, whereas extended outer discs with little or no star formation tend to be stable and unshielded on average.
We explore what drives the transition between these two regimes, specifically whether discs first meet the conditions for self-shielding (parameterized by dust optical depth, $tau$) or gravitational instability (parameterized by a modified version of Toomres instability parameters, $Q_{rm thermal}$, which quantifies the stability of a gas disc that is thermally supported at $T=10^4$ K). We first introduce a new metric formed by the product of these quantities, $Q_{rm thermal}tau$, which indicates whether the conditions for disk instability or self-shielding are easier to meet in a given region of a galaxy, and we discuss how $Q_{rm thermal}tau$ can be constrained even in the absence of direct gas information. We then analyse a sample of 13 galaxies with resolved gas measurements and find that on average galaxies will reach the threshold for disk instabilities ($Q_{rm thermal}<1$) before reaching the threshold for self-shielding ($tau>1$). Using integral field spectroscopic observations of a sample of 236 galaxies from the MaNGA survey, we find that the value of $Q_{rm thermal}tau$ in star-forming discs is consistent with similar behavior. These results support a scenario where disc fragmentation and collapse occurs before self-shielding, suggesting that gravitational instabilities are the primary condition for widespread star formation in galaxy discs. Our results support similar conclusions based on recent galaxy simulations.
We present an analysis of the global and spatially-resolved Kennicutt-Schmidt (KS) star formation relation in the FIRE (Feedback In Realistic Environments) suite of cosmological simulations, including halos with $z = 0$ masses ranging from $10^{10}$
-- $10^{13}$ M$_{odot}$. We show that the KS relation emerges and is robustly maintained due to the effects of feedback on local scales regulating star-forming gas, independent of the particular small-scale star formation prescriptions employed. We demonstrate that the time-averaged KS relation is relatively independent of redshift and spatial averaging scale, and that the star formation rate surface density is weakly dependent on metallicity and inversely dependent on orbital dynamical time. At constant star formation rate surface density, the `Cold & Dense gas surface density (gas with $T < 300$~K and $n > 10$~cm$^{-3}$, used as a proxy for the molecular gas surface density) of the simulated galaxies is $sim$0.5~dex less than observed at $sim$kpc scales. This discrepancy may arise from underestimates of the local column density at the particle-scale for the purposes of shielding in the simulations. Finally, we show that on scales larger than individual giant molecular clouds, the primary condition that determines whether star formation occurs is whether a patch of the galactic disk is thermally Toomre-unstable (not whether it is self-shielding): once a patch can no longer be thermally stabilized against fragmentation, it collapses, becomes self-shielding, cools, and forms stars, regardless of epoch or environment.
