Role of cosmic rays in the circumgalactic medium
Abstract
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.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- February 2016
- DOI:
- 10.1093/mnras/stv2641
- arXiv:
- arXiv:1511.05144
- Bibcode:
- 2016MNRAS.456..582S
- Keywords:
-
- methods: numerical;
- cosmic rays;
- galaxies: formation;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 22 pages, 14 figures, MNRAS in press