Constraining Cosmic-ray Transport with Observations of the Circumgalactic Medium

Understand

Recent theoretical studies predict that the circumgalactic medium (CGM) around low-redshift, $\sim L_*$ galaxies could have substantial nonthermal pressure support in the form of cosmic rays.

• However, 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 novel observational constraint for calculating the lower limit of the radially-averaged, effective cosmic-ray transport rate, $\kappa_{\rm min}^{\rm eff}$.
• Under a wide range of assumptions (so long as cosmic rays do not lose a significant fraction of their energy in the galactic disk, regardless of whether the cosmic-ray pressure is important or not in the CGM), we demonstrate a well-defined relationship between $\kappa_{\rm min}^{\rm eff}$ and three observable galaxy properties: the total hydrogen column density, the average star formation rate, and the gas circular velocity.