Thermal Instability in the CGM of L* Galaxies: Testing “Precipitation” Models with the FIRE Simulations

Esmerian et al., available on arXiv

Abstract: We examine the thermodynamic state and cooling of the low-z Circum-Galactic Medium (CGM) in five FIRE-2 galaxy formation simulations of Milky Way-mass galaxies. We find that the CGM in these simulations is generally multiphase and dynamic, with a wide spectrum of largely nonlinear density perturbations sourced by the accretion of gas from the IGM and winds from both the central and satellite galaxies. We investigate the origin of the multiphase structure of the CGM with a particle tracking analysis and find that most of the low entropy gas has cooled from the hot halo as a result of thermal instability triggered by these perturbations. The ratio of cooling to free-fall timescales tcool/tff in the hot component of the CGM spans a wide range ~1-100 at a given radius, but exhibits approximately constant median values ~5-20 at all radii 0.1Rvir < r < Rvir. These are similar to the ~10-20 value typically adopted as the thermal instability threshold in “precipitation” models of the ICM. Consequently, a one-dimensional model based on the assumption of a constant tcool/tff and hydrostatic equilibrium approximately reproduces the simulation number density and entropy profiles, but only if it assumes the metallicity profiles taken directly from the simulations. We explicitly show that the tcool/tff value of a gas parcel in the hot component of the CGM does not predict its probability of cooling and subsequently accreting onto the central galaxy. This suggests that the value of tcool/tff is a poor predictor of thermal stability in gaseous halos in which large-amplitude density perturbations are prevalent.