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.

9 pages, 2 figures, accepted to MNRAS

References in corpus (10)

• The COS-Halos Survey: Physical Conditions and Baryonic Mass in the Low-Redshift Circumgalactic Medium
• Global cosmic-ray related luminosity and energy budget of the Milky Way
• Gamma-ray sky points to radial gradients in cosmic-ray transport
• Cosmic ray-driven galactic winds: streaming or diffusion?
• Winds, Clumps, and Interacting Cosmic Rays in M82
• Implications of current nuclear cross sections on secondary cosmic rays with the upcoming DRAGON2 code
• Fermi-LAT Observations of High- and Intermediate-Velocity Clouds: Tracing Cosmic Rays in the Halo of the Milky Way
• Search for gamma-ray emission from star-forming galaxies with Fermi LAT
• The radio continuum perspective on cosmic-ray transport in external galaxies
• Interpreting the relation between the gamma-ray and infrared luminosities of star-forming galaxies

Cited by in corpus (10)

• High resolution modeling of [CII], [CI], [OIII] and CO line emission from the ISM and CGM of a star forming galaxy at z ~ 6.5
• The Effect of Cosmic Rays on the Observational Properties of the CGM
• Diverse dark matter profiles in FIRE dwarfs: black holes, cosmic rays and the cusp-core enigma
• A Simple Sub-Grid Model For Cosmic Ray Effects on Galactic Scales
• Lévy Flights and Leaky Boxes: Anomalous Diffusion of Cosmic Rays
• Strong Evidence for Cosmic Ray-Supported $\sim$L${}^{\ast }$ Galaxy Halos via X-ray \& tSZ Constraints
• Constraining cosmic ray transport models using circumgalactic medium properties and observables
• Effects of Varied Cosmic Ray Feedback from AGN on Massive Galaxy Properties
• 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