The impact of cosmic rays on dynamical balance and disk-halo interaction in Lstar disk galaxies

Abstract

Cosmic rays (CRs) are an important component in the interstellar medium (ISM), but their effect on the dynamics of the disk-halo interface (< 10 kpc from the disk) is still unclear. We study the influence of CRs on the gas above the disk with highresolution FIRE-2 cosmological simulations of late-type L galaxies at redshift z ∼ 0. We compare runs with and without CR feedback (with constant anisotropic diffusion κ ∼ 3 × 10 29 cm 2 /s and streaming). Our simulations capture the relevant disk halo interactions, including outflows, inflows, and galactic fountains. Extra-planar gas in all of the runs satisfies dynamical balance, where total pressure balances the weight of the overlying gas. While the kinetic pressure from non-uniform motion ( 1 kpc scale) dominates in the midplane, thermal and bulk pressures (or CR pressure if included) take over at large heights. We find that with CR feedback, (1) the warm (∼ 10 4 K) gas is slowly accelerated by CRs; (2) the hot (> 5 × 10 5 K) gas scale height is suppressed;(3) the warm-hot (2 × 10 4 − 5 × 10 5 K) medium becomes the most volume-filling phase in the disk-halo interface. We develop a novel conceptual model of the near-disk gas dynamics in low-redshift L galaxies: with CRs, the disk-halo interface is filled with CR-driven warm winds and hot super-bubbles that are propagating into the CGM with a small fraction falling back to the disk. Without CRs, most outflows from hot superbubbles are trapped by the existing hot halo and gravity, so typically they form galactic fountains.

Cosmic-Ray Transport in Varying Galactic Environments

“…Our predictions are in qualitative agreement with the results of recent simulations of Milky Way-like galaxies including CRs (Chan et al 2021). Similar to our approach in Section 4.1, Chan et al (2021) quantify gravitational weight and momentum flux differences ("vertical support") from different pressure components as a function of the height from the disk. The momentum flux profiles of CRs in their simulations (see their Fig.…”

“…2) are flatter than in our simulations (Figure 8). This is mainly because Chan et al (2021) adopt a spatially-constant diffusion coefficient of κ = 3 × 10 29 cm 2 s −1 , which is more than an order of magnitude larger than the average diffusion coefficient in the extra-planar regions of our models (Figure 5). Nevertheless, they also find that CRs can become dynamically dominant beyond a few kpc from the midplane.…”

Gas infall and radial transport in cosmological simulations of milky way-mass discs

“…It is worth noting that while the abo v e is true for the net accretion, the total gas infall rates are significantly higher (see Fig. 15 ) because of outflowing gas component caused by the oscillatory motion and CR-dri ven outflo ws from galaxies (Hopkins et al 2021a ;Chan et al 2021 ). Goldbaum, Krumholz & Forbes ( 2015 similarly studied radial mass fluxes in simulations of galactic discs with and without stellar feedback.…”

FIRE-3: Updated Stellar Evolution Models, Yields, & Microphysics and Fitting Functions for Applications in Galaxy Simulations

“…While our focus here is on the "default" FIRE-3 methods, we summarize the default CR implementation for completeness. In FIRE-2 simulations with explicit CR dynamics (Su et al 2019(Su et al , 2020(Su et al , 2021Chan et al 2019Chan et al , 2021Hopkins et al 2020bJi et al 2020Ji et al , 2021Trapp et al 2021), we treated CRs with a simplified approximation developed in Chan et al (2019): we evolved just the total CR energy density as a relativistic fluid obeying a simple two-moment equation with a somewhat ad-hoc "streaming plus diffusion" approximation. In FIRE-3 simulations including CR dynamics, our treatment of CRs is updated to match that presented in Hopkins et al (2021c): we evolve the full CR distribution function/spectrum for multiple species with a recently-derived rigorous two-moment formulation including all terms up to leading order in O(u/c) (where u is the MHD fluid velocity).…”