2025/06/30 by Lucia Armillotta, Eve C. Ostriker, Armillotta, Lucia +3 · 7 citations
Physics and Astronomy · #Astrophysics and Cosmic Phenomena #Astrophysics and Star Formation Studies #Astrophysics of Galaxies (astro-ph.GA) #Dark Matter and Cosmic Phenomena #FOS: Physical sciences #High Energy Astrophysical Phenomena (astro-ph.HE)
paper · pdf · doi:10.48550/arxiv.2507.00120
openalex publication_date 2025/06/30 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
We investigate the transport of spectrally resolved cosmic ray (CR) protons with kinetic energies between 1-100 GeV within the dynamic, multiphase interstellar medium (ISM), using a two-moment CR fluid solver applied to a TIGRESS MHD simulation with conditions similar to the solar neighborhood. Our CR transport prescription incorporates space- and momentum-dependent CR scattering coefficients σ=κ-1, computed from the local balance between streaming-driven Alfvèn wave growth and damping processes. We find that advection combines with momentum-dependent diffusion to produce a CR distribution function f(p)∝~p-γ with γ≈4.6 that agrees with observations, steepened from an injected power law slope γinj=4.3. The CR pressure is uniform in the highly diffusive, mostly neutral midplane region, but decreases exponentially in the ionized extraplanar region where scattering is efficient. To interpret these numerical results, we develop a two-zone analytic model that captures and links the two (physically and spatially) distinct regimes of CR transport in the multiphase, dynamic ISM. At low momenta, CR transport is dominated by gas advection, while at high momenta, both advection and diffusion contribute. At high momentum, the analytic prediction for the spectral slope approaches γ=(4/3)γinj-1, and the predicted scaling of grammage with momentum is X∝ p1-γinj/3, consistent with the simulations. These results support a physical picture in which CRs are confined within the neutral midplane by the surrounding ionized gas, with their escape regulated by both the CR scattering rate in the ionized extraplanar gas and the velocity and Alfvén speed of that gas, at effective speed vc,eff≈(1/2)[κ_∥~d(v+vA,i)/dz]1/2.