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Testing physical models for cosmic ray transport coefficients on galactic scales: self-confinement and extrinsic turbulence at ∼GeV energies

Hopkins, Philip F. and Squire, Jonathan and Chan, T. K. and Quataert, Eliot and Ji, Suoqing and Kereš, Dušan and Faucher-Giguère, Claude-André (2021) Testing physical models for cosmic ray transport coefficients on galactic scales: self-confinement and extrinsic turbulence at ∼GeV energies. Monthly Notices of the Royal Astronomical Society, 501 (3). pp. 4184-4213. ISSN 0035-8711. doi:10.1093/mnras/staa3691.

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The microphysics of ∼ GeV cosmic ray (CR) transport on galactic scales remain deeply uncertain, with almost all studies adopting simple prescriptions (e.g. constant diffusivity). We explore different physically motivated, anisotropic, dynamical CR transport scalings in high-resolution cosmological Feedback In Realistic Environment (FIRE) simulations of dwarf and ∼L* galaxies where scattering rates vary with local plasma properties motivated by extrinsic turbulence (ET) or self-confinement (SC) scenarios, with varying assumptions about e.g. turbulent power spectra on un-resolved scales, Alfvén-wave damping, etc. We self-consistently predict observables including γ-rays (Lγ), grammage, residence times, and CR energy densities to constrain the models. We demonstrate many non-linear dynamical effects (not captured in simpler models) tend to enhance confinement. For example, in multiphase media, even allowing arbitrary fast transport in neutral gas does not substantially reduce CR residence times (or L_γ), as transport is rate-limited by the ionized WIM and ‘inner CGM’ gaseous halo (10⁴–10⁶ K gas within ≲10−30 kpc), and L_γ can be dominated by trapping in small ‘patches’. Most physical ET models contribute negligible scattering of ∼1–10 GeV CRs, but it is crucial to account for anisotropy and damping (especially of fast modes) or else scattering rates would violate observations. We show that the most widely assumed scalings for SC models produce excessive confinement by factors ≳100 in the warm ionized medium (WIM) and inner CGM, where turbulent and Landau damping dominate. This suggests either a breakdown of quasi-linear theory used to derive the CR transport parameters in SC, or that other novel damping mechanisms dominate in intermediate-density ionized gas.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper ItemGIZMO Code ItemFIRE project
Hopkins, Philip F.0000-0003-3729-1684
Squire, Jonathan0000-0001-8479-962X
Chan, T. K.0000-0003-2544-054X
Quataert, Eliot0000-0001-9185-5044
Ji, Suoqing0000-0001-9658-0588
Kereš, Dušan0000-0002-1666-7067
Faucher-Giguère, Claude-André0000-0002-4900-6628
Additional Information:© 2020 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 ( Accepted 2020 November 5. Received 2020 October 5; in original form 2020 February 5. Published: 28 November 2020. We thank the anonymous referee for helpful suggestions. Support for PFH was provided by NSF Collaborative Research Grants 1715847 and 1911233, NSF CAREER grant 1455342, NASA grants 80NSSC18K0562, and JPL 1589742. CAFG was supported by NSF 1517491, 1715216, and CAREER 1652522; NASA 17-ATP17-0067; and by a Cottrell Scholar Award. DK was supported by NSF grant AST-1715101 and the Cottrell Scholar Award. Numerical calculations were run on the Caltech compute cluster ‘Wheeler’, allocations from XSEDE TG-AST130039 and PRAC NSF.1455342 supported by the NSF, and NASA HEC SMD-16-7592. Data used in this work were hosted on facilities supported by the Scientific Computing Core at the Flatiron Institute, a division of the Simons Foundation. Data Availability Statement: The data supporting the plots within this article are available on reasonable request to the corresponding author. A public version of the GIZMO code is available at Additional data including simulation snapshots, initial conditions, and derived data products are available at
Group:Astronomy Department, TAPIR
Funding AgencyGrant Number
Cottrell Scholar of Research CorporationUNSPECIFIED
Flatiron InstituteUNSPECIFIED
Simons FoundationUNSPECIFIED
Subject Keywords:instabilities – plasmas – cosmic rays – ISM: structure – galaxies: evolution – gamma-rays: galaxies
Issue or Number:3
Record Number:CaltechAUTHORS:20200309-153702109
Persistent URL:
Official Citation:Philip F Hopkins, Jonathan Squire, T K Chan, Eliot Quataert, Suoqing Ji, Dušan Kereš, Claude-André Faucher-Giguère, Testing physical models for cosmic ray transport coefficients on galactic scales: self-confinement and extrinsic turbulence at ∼GeV energies, Monthly Notices of the Royal Astronomical Society, Volume 501, Issue 3, March 2021, Pages 4184–4213,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:101798
Deposited By: Tony Diaz
Deposited On:09 Mar 2020 23:28
Last Modified:02 Sep 2021 20:30

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