Synchrotron Signatures of Cosmic Ray Transport Physics in Galaxies

TL;DR

This study uses cosmological simulations to compare synchrotron emission signatures of different cosmic ray transport models, revealing observable differences that can constrain the physics of cosmic ray propagation in galaxies.

Contribution

It introduces a simulation-based approach to distinguish cosmic ray transport mechanisms via synchrotron emission, highlighting observable signatures that can inform galaxy formation models.

Findings

SC models produce brighter, smoother synchrotron emission.

Differences in morphology and magnetic fields distinguish transport models.

Results suggest SC is unlikely to dominate in typical star-forming galaxies.

Abstract

Cosmic rays (CRs) may drive outflows and alter the phase structure of the circumgalactic medium, with potentially important implications on galaxy formation. However, these effects ultimately depend on the dominant mode of transport of CRs within and around galaxies, which remains highly uncertain. To explore potential observable constraints on CR transport, we investigate a set of cosmological FIRE-2 CR-MHD simulations of L$_{\ast}$ galaxies which evolve CRs with transport models motivated by self-confinement (SC) and extrinsic turbulence (ET) paradigms. To first order, the synchrotron properties diverge between SC and ET models due to a CR physics driven hysteresis. SC models show a higher tendency to undergo `ejective' feedback events due to a runaway buildup of CR pressure in dense gas due to the behavior of SC transport scalings at extremal CR energy densities. The corresponding CR wind-driven hysteresis results in brighter, smoother, and more extended synchrotron emission in SC runs relative to ET and constant diffusion runs. The differences in synchrotron arise from different morphology, ISM gas and \textbf{B} properties, potentially ruling out SC as the dominant mode of CR transport in typical star-forming L$_{\ast}$ galaxies, and indicating the potential for non-thermal radio continuum observations to constrain CR transport physics.