Testing Physical Models for Cosmic Ray Transport Coefficients on Galactic Scales: Self-Confinement and Extrinsic Turbulence at GeV Energies. (arXiv:2002.06211v1 [astro-ph.HE])

Testing Physical Models for Cosmic Ray Transport Coefficients on Galactic Scales: Self-Confinement and Extrinsic Turbulence at GeV Energies. (arXiv:2002.06211v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Hopkins_P/0/1/0/all/0/1">Philip F. Hopkins</a> (Caltech), <a href="http://arxiv.org/find/astro-ph/1/au:+Squire_J/0/1/0/all/0/1">Jonathan Squire</a> (Otago), <a href="http://arxiv.org/find/astro-ph/1/au:+Chan_T/0/1/0/all/0/1">T. K. Chan</a> (ICC Durham), <a href="http://arxiv.org/find/astro-ph/1/au:+Quataert_E/0/1/0/all/0/1">Eliot Quataert</a> (Berkeley), <a href="http://arxiv.org/find/astro-ph/1/au:+Ji_S/0/1/0/all/0/1">Suoqing Ji</a> (Caltech), <a href="http://arxiv.org/find/astro-ph/1/au:+Keres_D/0/1/0/all/0/1">Dusan Keres</a> (UCSD), <a href="http://arxiv.org/find/astro-ph/1/au:+Faucher_Giguere_C/0/1/0/all/0/1">Claude-Andre Faucher-Giguere</a> (Northwestern)

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 ~$L_{ast}$ 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 $gamma$-rays ($L_{gamma}$),
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 $L_{gamma}$), as transport is rate-limited by the ionized
WIM and ‘inner CGM’ gaseous halo ($10^{4}-10^{6}$ K gas within 10-30 kpc), and
$L_{gamma}$ 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.

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 ~$L_{ast}$ 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 $gamma$-rays ($L_{gamma}$),
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 $L_{gamma}$), as transport is rate-limited by the ionized
WIM and ‘inner CGM’ gaseous halo ($10^{4}-10^{6}$ K gas within 10-30 kpc), and
$L_{gamma}$ 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.

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