Constraining Cosmic-ray Transport with Observations of the Circumgalactic Medium

Abstract

Recent theoretical studies predict that the circumgalactic medium (CGM) around low-redshift, $\sim L_{\ast }$ 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, ${\kappa }_{\min }^{\mathrm{eff}}$. 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 ${\kappa }_{\min }^{\mathrm{eff}}$ 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 FIRE-2 galaxy simulations with a variety of cosmic-ray transport physics to demonstrate that our analytic model of ${\kappa }_{\min }^{\mathrm{eff}}$ is a robust lower limit of the true cosmic-ray transport rate. We then apply our new model to calculate ${\kappa }_{\min }^{\mathrm{eff}}$ for galaxies in the COS-Halos sample, and confirm this already reveals strong evidence for an effective transport rate which rises rapidly away from the interstellar medium to values ${\kappa }_{\min }^{\mathrm{eff}}\gtrsim 10^{30-31}{\mathrm{cm}}^2{\mathrm{s}}^{-1}$ (corresponding to anisotropic streaming velocities of $v_{\mathrm{eff}}^{\mathrm{stream}}\gtrsim 1000\mathrm{km}{\mathrm{s}}^{-1}$) in the diffuse CGM, at impact parameters larger than $50-100$\,kpc. We discuss how future observations can provide qualitatively new constraints in our understanding of cosmic rays in the CGM and intergalactic medium.

arXiv comment9 pages, 2 figures, accepted to MNRAS

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• A Simple Sub-Grid Model For Cosmic Ray Effects on Galactic Scales
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• Effect of cosmic rays and ionizing radiation on observational ultraviolet plasma diagnostics in the circumgalactic medium
• Time-Dependent Cosmic Ray Halos from Bursty Star Formation and Active Galactic Nuclei: Semi-Analytic Formalism and Galaxy Formation Implications