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Published May 2023 | Published
Journal Article Open

Constraining cosmic ray transport with observations of the circumgalactic medium


Recent theoretical studies predict that the circumgalactic medium (CGM) around low-redshift, ∼L* galaxies could have substantial non-thermal 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, κ_(eff)^(min). 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 κ_(eff)^(min) 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 κ_(eff)^(min) is a robust lower limit of the true cosmic ray transport rate. We then apply our new model to calculate κ_(eff)^(min) 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 κ_(eff)^(min) ≳ 10^(30-31) cm² 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 qualitatively new constraints in our understanding of cosmic rays in the CGM and intergalactic medium.

Additional Information

© 2023 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model). The authors would like to thank Gina Panopoulou for her insightful comments on this work. ISB was supported by the DuBridge Postdoctoral Fellowship at Caltech. Support for PFH and co-authors was provided by NSF Research Grants 1911233 and 20009234, NSF CAREER grant 1455342, and NASA grants 80NSSC18K0562 and HST-AR-15800.001-A. Numerical calculations were run on the Caltech compute cluster 'Wheeler', allocations FTA-Hopkins supported by the NSF and TACC, and NASA HEC SMD-16-7592. The data used in this work were, in part, hosted on facilities supported by the Scientific Computing Core at the Flatiron Institute, a division of the Simons Foundation. DATA AVAILABILITY. A public version of the GIZMO code is available at http://www.tapir.caltech.edu/~phopkins/Site/GIZMO.html.

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August 22, 2023
August 22, 2023