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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.
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This content will become publicly available on July 3, 2026
Cosmic Ray Mediated Thermal Fronts in the Warm-hot Circumgalactic Medium
Abstract We investigate the 1D plane-parallel front connecting the warm (104K) and hot (106K) phases of the circumgalactic medium (CGM), focusing on the influence of cosmic rays (CRs) in shaping these transition layers. We find that cosmic rays dictate the thermal balance while other fluxes (thermal conduction, radiative cooling, and gas flow) adjust to compensate. We compute column densities and ratios for the transition-temperature ions Siiv, Civ, Ovi, and Nv, and compare them with observational data. While most models struggle to simultaneously reproduce the observed Siiv/Civand Civ/Oviratios, a subset with intermediate magnetic field strength (e.g.,B= 20μG) shows overlap with the data, although we make no claims for their uniqueness. These discrepancies suggest that the models perform better at reproducing higher-temperature ions but underestimate the contribution from cooler, photoionized regions. Compared to models without CRs, CR-mediated fronts in sufficiently strong magnetic fields produce broader transition layers and higher ion ratios, indicating that CRs can significantly alter the thermal and ionization structure of the CGM. Our results suggest that CR heating may help explain some observed ion columns under specific conditions, though additional physics may be needed for full agreement with observations.
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- Award ID(s):
- 2007323
- PAR ID:
- 10634482
- Publisher / Repository:
- IOP
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 987
- Issue:
- 2
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 132
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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