Properties of the circumgalactic medium in cosmic ray-dominated galaxy haloes

Abstract

We investigate the impact of cosmic rays (CRs) on the circumgalactic medium (CGM) in FIRE-2 simulations, for ultra-faint dwarf through Milky Way (MW)-mass haloes hosting star-forming (SF) galaxies. Our CR treatment includes injection by supernovae, anisotropic streaming and diffusion along magnetic field lines, and collisional and streaming losses, with constant parallel diffusivity $\kappa \sim 3\times 10^{29}\, \mathrm{cm^2\ s^{-1}}$ chosen to match γ-ray observations. With this, CRs become more important at larger halo masses and lower redshifts, and dominate the pressure in the CGM in MW-mass haloes at z ≲ 1–2. The gas in these ‘CR-dominated’ haloes differs significantly from runs without CRs: the gas is primarily cool (a few ${\sim}10^{4}\,$ K), and the cool phase is volume-filling and has a thermal pressure below that needed for virial or local thermal pressure balance. Ionization of the ‘low’ and ‘mid’ ions in this diffuse cool gas is dominated by photoionization, with O vi columns ${\gtrsim}10^{14.5}\, \mathrm{cm^{-2}}$ at distances ${\gtrsim}150\, \mathrm{kpc}$. CR and thermal gas pressure are locally anticorrelated, maintaining total pressure balance, and the CGM gas density profile is determined by the balance of CR pressure gradients and gravity. Neglecting CRs, the same haloes are primarily warm/hot ($T\gtrsim 10^{5}\,$K) with thermal pressure balancing gravity, collisional ionization dominates, O vi columns are lower and Ne viii higher, and the cool phase is confined to dense filaments in local thermal pressure equilibrium with the hot phase.

The Impact of Cosmic Rays on Thermal Instability in the Circumgalactic Medium

“…In a cosmic-ray-pressure-dominated halo, cool, warm, and hot gas phases can exist at the same gas density (Ji et al 2020). Although there are few observational constraints of cosmic-ray pressures in the CGM, simulations predict that a cosmic-ray-pressure-dominated halo could be consistent with existing γ-ray observations (Chan et al 2019;Ji et al 2020).…”

“…Galaxy simulations that include cosmic-ray physics also find that cosmic-ray pressure supports cold CGM gas at low densities (Salem et al 2016;Butsky & Quinn 2018;Buck et al 2020). In a cosmic-ray-pressure-dominated halo, cool, warm, and hot gas phases can exist at the same gas density (Ji et al 2020). Although there are few observational constraints of cosmic-ray pressures in the CGM, simulations predict that a cosmic-ray-pressure-dominated halo could be consistent with existing γ-ray observations (Chan et al 2019;Ji et al 2020).…”

“…Cosmic rays are likely an important source of energy in the CGM. In the Milky Way ISM, cosmic-ray energy is roughly in equipartition with thermal and magnetic energies (Ginzburg & Ptuskin 1985;Boulares & Cox 1990), and many recent simulations have shown that cosmic rays launch far-reaching galactic outflows (Uhlig et al 2012;Booth et al 2013;Girichidis et al 2016;Simpson et al 2016;Ruszkowski et al 2017;Bustard et al 2020;Jana et al 2020), altering the phase structure of the CGM (Salem et al 2016;Butsky & Quinn 2018;Buck et al 2020;Ji et al 2020). Galaxy-scale simulations that include cosmic rays find that cosmic-ray pressure may even be the dominant source of pressure in the CGM (Girichidis et al 2018;Ji et al 2020).…”

Project AMIGA: The Circumgalactic Medium of Andromeda*

“…The CR simulation assumes a diffusion coefficient || k =3 10 29 cm −2 s −1 , which was calibrated to be consistent with observational constraints from γ−ray emission of the MW and some other nearby galaxies (Chan et al 2019;Hopkins et al 2020). Ji et al (2020) showed that CRs can potentially provide a large or even dominant nonthermal fraction of the total pressure support in the CGM of low-redshift ∼L * galaxies. As a result, in the fiducial CR run analyzed here, the volume-filling CGM is much cooler (∼10 4 -10 5 K) and is thus photoionized in regions where in the run without CRs it is filled with hot gas that is more collisionally ionized.…”

“…(1) Comparison with FIRE-2 Zoom Simulations We first compare our observations with column densities modeled using cosmological zoom-in simulations from the FIRE project. 31 Details of the simulation setup and CGM modeling methods are presented in Ji et al (2020). Briefly, the outputs analyzed here are FIRE-2 simulations evolved with the GIZMO code using the meshless finite mass (MFM) solver (Hopkins 2015).…”