Constraining cosmic ray transport with observations of the circumgalactic medium

Recent theoretical studies predict that the circumgalactic medium (CGM) around low-redshift, similar to L galaxies could have substantial non-thermal pressure support in the form of cosmic rays. Ho we ver, these predictions are sensitive to the specific model of cosmic ray transport employed, which is theoretically and observationally underconstrained. In this work, we propose a no v el observational constraint for calculating the lower limit of the radially averaged, effective cosmic ray transport rate, kappa(min)(eff) . Under a wide range of assumptions (so long as cosmic rays do not lose a significant fraction of their energy in the galactic disc, regardless of whether the cosmic ray pressure is important or not in the CGM), we demonstrate a well-defined relationship between kappa(min)(eff) and three observable galaxy properties: the total hydrogen column density, the average star formation rate, and the gas circular velocity. We use a suite of Feedback in Realistic Environments 2 galaxy simulations with a variety of cosmic ray transport physics to demonstrate that our analytical model of kappa(min)(eff) is a robust lower limit of the true cosmic ray transport rate. We then apply our new model to calculate kappa(min)(eff) for galaxies in the COS-Halos sample, and confirm this already reveals strong evidence for an effective transport rate that rises rapidly away from the interstellar medium to values kappa(min)(eff) ? 10(30 -31) cm(2) s( -1) (corresponding to anisotropic streaming velocities of v(stream)(eff) ? 1000 km s(-1)) in the diffuse CGM, at impact parameters larger than 50-100 kpc. We discuss how future observations can provide qualitati vely ne w constraints in our understanding of cosmic rays in the CGM and intergalactic medium.

Automated summary

Recent theoretical studies suggest that the circumgalactic medium (CGM) surrounding low-redshift L galaxies may have significant non-thermal pressure support in the form of cosmic rays. However, the specific model of cosmic ray transport employed in these predictions lacks theoretical and observational constraints. This work proposes a new observational constraint for calculating the lower limit of the effective cosmic ray transport rate, demonstrating a relationship between the transport rate and observable galaxy properties. The findings suggest strong evidence for an effective transport rate in the diffuse CGM.