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

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 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 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.

Which AGN jets quench star formation in massive galaxies?

“…Streaming and diffusion are fully-anisotropic along magnetic field lines. In Chan et al (2019) and Hopkins et al (2019Hopkins et al ( , 2021c, we showed that matching observed γ -ray luminosities in simulations with the physics above requires κ CR ∼ 10 29 cm 2 s −1 , in good agreement with detailed CR transport models that include an extended gaseous halo around the Galaxy (see e.g. Strong & Moskalenko 1998;Strong et al 2010;Trotta et al 2011), so we adopt this as our fiducial value.…”

“…Because the CR energy density is much larger than kinetic energy density, the CR jet cocoon covers a wider angle (as expected), and can therefore more efficiently suppress inflow. If the CR losses are negligible and CRs become quasi-isotropic, with an effective isotropically averaged diffusivity κ (which includes streaming+advection), then as shown in various studies (Butsky & Quinn 2018;Hopkins et al 2019Hopkins et al , 2021aJi et al 2020) the CR pressure for steady-state injection is P CR (r) ∼ ĖCR /12π κ r. Comparing the outward acceleration ρ −1 ∇P to gravity (∼ v 2 c /r), where v c is the circular velocity, CR pressure alone can support the gas if ĖCR 10 43 erg s −1 10 29 κ n gas 0.01 cm −3 r 30 kpc v c 500 km s −1 2 (15) (where we recall the cooling radius is ∼ 30 kpc here and the diffusivity used here is 10 29 cm 2 s −1 ; see Chan et al 2019;Hopkins et al 2019Hopkins et al , 2021c. This roughly explains the required CR energetics we find in our simulations, as well as the radii/densities where CR pressure dominates for a given ĖCR .…”

Virial shocks are suppressed in cosmic ray-dominated galaxy haloes

“…For instance, when CRs enter the CGM where gas number densities, magnitudes of magnetic fields and strengths of turbulence are lower, CRs might become less confined and escape from galaxy haloes more quickly than what we see in our CR+ runs. In Hopkins et al (2020b) we have already shown that observationally allowed diffusion coefficients that scales with physical properties of the gas typically leads to properties of galaxies intermediate between MHD and CR+. As a consequence, the level of CR pressure dominance in the CGM and the impact of CRs on the formation of virial shocks in our simulations might change with more realistic CR transport treatments.…”

Towards First-principle Characterization of Cosmic-ray Transport Coefficients from Multi-scale Kinetic Simulations

“…As a result, there are substantial degrees of freedom in setting the diffusion coefficients and streaming speeds, which can lead to dramatically different outcomes and hence substantial uncertainties (e.g. Hopkins et al 2021c).…”