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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; Squire, Jonathan; Chan, TK; Quataert, Eliot; Ji, Suoqing; Kereš, Dušan; Faucher-Giguère, Claude-André

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


Authors

Philip F Hopkins

Jonathan Squire

Eliot Quataert

Suoqing Ji

Dušan Kereš

Claude-André Faucher-Giguère



Abstract

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 Lγ), as transport is rate-limited by the ionized WIM and ‘inner CGM’ gaseous halo (104–106 K gas within ≲10−30 kpc), and Lγ 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.

Citation

Hopkins, P. F., Squire, J., Chan, T., Quataert, E., Ji, S., Kereš, D., & Faucher-Giguère, C. (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), 4184-4213. https://doi.org/10.1093/mnras/staa3691

Journal Article Type Article
Acceptance Date Nov 5, 2020
Online Publication Date Nov 28, 2020
Publication Date 2021-03
Deposit Date Jul 1, 2021
Publicly Available Date Mar 28, 2024
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 501
Issue 3
Pages 4184-4213
DOI https://doi.org/10.1093/mnras/staa3691

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Copyright Statement
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2020 the Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.





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