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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 (2020) Testing Physical Models for Cosmic Ray Transport Coefficients on Galactic Scales: Self-Confinement and Extrinsic Turbulence at GeV Energies. . (Unpublished)

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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 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, Alfven-wave damping, etc. We self-consistently predict observables including γ-rays (L_γ), 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 multi-phase 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 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:Report or Paper (Discussion Paper)
Related URLs:
URLURL TypeDescription Paper
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:Support for PFH was provided by NSF Collaborative Research Grants 1715847 & 1911233, NSF CAREER grant 1455342, and 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.
Group:Astronomy Department, TAPIR
Funding AgencyGrant Number
Cottrell Scholar of Research CorporationUNSPECIFIED
Flatiron InstituteUNSPECIFIED
Simons FoundationUNSPECIFIED
Subject Keywords:cosmic rays—plasmas—instabilities—gamma-rays: galaxies—galaxies: evolution—ISM: structure
Record Number:CaltechAUTHORS:20200309-153702109
Persistent URL:
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:09 Mar 2020 23:28

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