2019/09/30 by Suoqing Ji, T. K. Chan, Cameron B. Hummels +6 · 1 citation
Physics and Astronomy · #astro-ph.GA #astro-ph.CO #astro-ph.HE
paper · pdf · doi:10.1093/mnras/staa1849
17 pages, 14 figures, published version on MNRAS
arxiv created 2020/08/06 · arxiv updated 2020/08/10
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 halos hosting star-forming (SF) galaxies. Our CR treatment includes injection by supernovae, anisotropic streaming and diffusion along magnetic field lines, collisional and streaming losses, with constant parallel diffusivity κ∼3×1029 \mathrmcm2 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 halos at z\lesssim 1-2. The gas in these "CR-dominated" halos differs significantly from runs without CRs: the gas is primarily cool (a few ∼104 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 photo-ionization, with O VI columns \gtrsim 1014.5 \mathrmcm-2 at distances \gtrsim 150 kpc. CR and thermal gas pressure are locally anti-correlated, 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 halos are primarily warm/hot (T\gtrsim 105 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.