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 10^12 Msolar 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 Zsolar, thanks to robust, persistent outflows
from the disk. The CR-inclusive CGMs also feature more diffuse gas at lower
temperatures, down to 10^4 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-Halos (for HI, SiIV, CIII and OVI) than the non-CR run. We also compare our
models to recent, preliminary observations of diffuse gamma-ray emission in
local group halos. For our lower CR-diffusion runs with kappa_CR in the range
0.3 to 1 x 10^28 cm^2/s, the CR enriched CGM produces an inconsistently high
level of gamma emission. But the model with a relatively high kappa_CR = 3 x
10^28 cm^2/s provided a gamma-ray luminosity consistent with the extra-galactic
gamma-ray background observed by FERMI and roughly consistent with preliminary
measures of the emission from M31's CGM.
