2020/02/29 by Philip F. Hopkins, Jonathan Squire, T K Chan +7 · 2 citations
Physics and Astronomy · #Anisotropy #Astrophysics #Astrophysics and Cosmic Phenomena #Cosmic ray #Dark Matter and Cosmic Phenomena #Galaxies: Formation, Evolution, Phenomena #Galaxy #Mechanics #Physics #Scattering #Turbulence #astro-ph.GA #astro-ph.HE #astro-ph.SR #physics.plasm-ph #physics.space-ph
paper · pdf · doi:10.1093/mnras/staa3691
published as MNRAS, 2021, 501, 3 · 29 pages, 12 figures. Replaced with published MNRAS version
openalex publication_date 2020/11/27 · arxiv created 2022/03/16 · arxiv updated 2022/03/18 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/06
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 (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 multi-phase 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 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.