The Effect of Cosmic Rays on the Observational Properties of the CGM

The circumgalactic medium (CGM) contains information on the cumulative effect of galactic outflows over time, generally thought to be caused by feedback from star formation and active galactic nuclei. Observations of such outflows via absorption in CGM gas of quasar sightlines show a significant amount of cold ($\lesssim 10^4 \; \rm{K}$) gas which cosmological simulations struggle to reproduce. Here, we use the adaptive mesh refinement hydrodynamical code RAMSES to investigate the effect of cosmic rays (CR) on the cold gas content of the CGM using three zoom realizations of a $z=1$ star-forming galaxy with supernova mechanical feedback: one with no CR feedback (referred to as no-CR), one with a medium CR diffusion coefficient $\kappa = 10^{28} \; \rm{cm^{2}\; s^{-1}}$ (CR$-\kappa_{\rm med}$), and one with a high rate of diffusion of $\kappa = 3\times10^{29} \; \rm{cm^{2}\; s^{-1}}$ (CR$-\kappa_{\rm high}$). We find that, for CR$-\kappa_{\rm med}$, the effects of CRs are largely confined to the galaxy itself as CRs do not extend far into the CGM. However, for CR$-\kappa_{\rm high}$, the CGM temperature is lowered and the amount of outflowing gas is boosted. Our CR simulations fall short of the observed Mg II covering fraction, a tracer of gas at temperatures $\lesssim 10^4 \; \rm{K}$, but the CR$-\kappa_{\rm high}$ simulation is more in agreement with covering fractions of C IV and O VI, which trace higher temperature gas.