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arXiv · 2509.03519

Dynamically Controlled Transport of GeV Cosmic Rays in Diverse Galactic Environments

Ronan Hix·Lucia Armillotta·Eve Ostriker·Chang-Goo Kim
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Abstract

We study transport of GeV cosmic rays (CRs) in a set of high-resolution TIGRESS magnetohydrodynamic simulations of the star-forming interstellar medium (ISM). Our local disk patch models sample a wide range of gas surface densities, gravitational potentials, and star formation rates (SFRs), and include a spiral arm simulation. Our approach incorporates CR advection by the background gas, streaming along the magnetic field limited by the local ion Alfv\'en speed, and diffusion relative to the Alfv\'en wave frame, with the diffusion coefficient set by the balance between streaming-driven Alfv\'en wave excitation and damping mediated by local gas properties. We find that dynamical transport mechanisms (streaming and advection) are almost solely responsible for GeV CR transport in the extra-planar regions of galaxies, while diffusion along the magnetic field dominates within the primarily-neutral ISM of galactic disks. We develop a simple 1D predictive model for the CR pressure $P_\mathrm{c}$, dependent only on injected CR flux and gas parameters. We demonstrate that the CR transport efficiency increases with increasing SFR, and provide a fit for the CR feedback yield $\Upsilon_\mathrm{c}~\equiv~P_\mathrm{c}/\Sigma_\mathrm{SFR}$ as a function of $\Sigma_\mathrm{SFR}$, the SFR surface density. We analyze lateral CR transport within the galactic disk, showing that CRs propagate away from feedback regions in spiral arms into interarm regions by a combination of gas advection and field-aligned transport. Lastly, we develop an empirical subgrid model for the CR scattering rate that captures the impacts of the multiphase ISM on CR transport without the numerical burden of full simulations.

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Ronan Hix, Lucia Armillotta, Eve Ostriker, Chang-Goo Kim. 2025-09-03. Dynamically Controlled Transport of GeV Cosmic Rays in Diverse Galactic Environments. https://arxiv.org/abs/2509.03519

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