Role of cosmic rays in the circumgalactic medium

We explore the impact of cosmic rays (CRs) on cosmological adaptive-mesh refinement simulations of a forming 1012 M⊙ halo, focusing on the circumgalactic medium (CGM), and its resulting low-redshift structure and composition. In contrast to a run with star formation and energetic feedback but no CRs, the CR-inclusive runs feature a CGM substantially enriched with CRs and with metals to roughly 0.1 Z⊙, thanks to robust, persistent outflows from the disc. The CR-inclusive CGMs also feature more diffuse gas at lower temperatures, down to 104 K, than the non-CR run, with diffuse material often receiving a majority of its pressure support from the CR proton fluid. We compare to recent observations of the CGM of L ∼ L* galaxies at low redshift, including UV absorption lines within background quasar spectra. The combination of metal-enriched, CR-driven winds and large swaths of CR pressure-supported, cooler diffuse gas leads to a CGM that provides a better match to data from COS-Haloes (for H i, Si iv, C iii and O vi) than the non-CR run. We also compare our models to recent, preliminary observations of diffuse gamma-ray emission in Local Group haloes. For our lower CR-diffusion runs with κCR ∈ {0.3, 1} × 1028 cm2 s−1, the CR-enriched CGM produces an inconsistently high level of gamma emission. But the model with a relatively high κCR = 3 × 1028 cm2 s−1 provided a gamma-ray luminosity consistent with the ‘extragalactic’ gamma-ray background observed by Fermi and roughly consistent with preliminary measures of the emission from M31's CGM.