Revisiting radio synchrotron diagnostics in star-forming galaxies
5 affiliations
- 1Max-Planck-Institut für Astrophysik (MPA) , Karl-Schwarzschild-Str. 1, D-85748 Garching ,
- 2Leibniz-Institut für Astrophysik Potsdam (AIP) , An der Sternwarte 16, D-14482 Potsdam ,
- 3Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik , Albert-Ueberle-Str. 2, D-69120 Heidelberg ,
- 4University of Zurich, Department of Astrophysics , Winterthurerstrasse 190, CH-8057 Zurich ,
- 5Cardiff Hub for Astrophysics Research and Technology, School of Physics and Astronomy, Cardiff University , Queen’s Buildings, Cardiff CF24 3AA ,
Abstract
Radio continuum observations are widely used to study cosmic ray (CR) electron populations and transport processes in star-forming galaxies, but their interpretation relies on several simplifying assumptions. Here, we revisit three common assumptions: that some vertical radio profiles can be explained by CR advection alone, that radio spectra directly trace the galaxy-wide CR electron spectrum, and that bremsstrahlung and Coulomb losses are negligible for radio-emitting electrons. We model radio emission using time-dependent CR electron evolution in a magnetohydrodynamical simulation of an isolated Milky Way-mass galaxy. CR electron spectra are evolved self-consistently along Lagrangian tracer particles with the Crest framework, including injection at supernova remnants, advection with the gas, and spatially and temporally varying radiative losses. We compare these results to commonly adopted steady-state models. We find that advection-only transport in self-consistently driven galactic winds fails to reproduce the extended vertical radio intensity profiles observed in edge-on galaxies, despite reproducing the observed steepening of spectral indices with height. This is because slowly accelerating winds keep electrons in strong cooling environments for too long. Matching observed radio haloes with advection alone requires unrealistically high mid-plane wind velocities, implying that additional transport or re-acceleration processes are required. Although galaxy-integrated CR electron spectra at radio-emitting energies are similar across models, the resulting synchrotron spectra differ systematically because radio emission is biased towards young electrons in dense, strongly magnetized regions. Finally, we show that bremsstrahlung and Coulomb losses significantly shape radio spectra even when their loss rate is subdominant and therefore cannot be neglected.
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BibTeX
@article{werhahn2026revisiting,
title = {{Revisiting radio synchrotron diagnostics in star-forming galaxies}},
author = {Maria Werhahn and Christoph Pfrommer and Philipp Girichidis and Joseph Whittingham and Léna Jlassi and Rüdiger Pakmor and Rebekka Bieri and Rainer Weinberger and Volker Springel and Freeke van de Voort},
journal = {Monthly Notices of the Royal Astronomical Society},
year = {2026},
month = oct,
doi = {10.1093/mnras/stag1871},
url = {https://academic.oup.com/mnras/article/doi/10.1093/mnras/stag1871/8859316},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Revisiting radio synchrotron diagnostics in star-forming galaxies
AU - Maria Werhahn
AU - Christoph Pfrommer
AU - Philipp Girichidis
AU - Joseph Whittingham
AU - Léna Jlassi
AU - Rüdiger Pakmor
AU - Rebekka Bieri
AU - Rainer Weinberger
AU - Volker Springel
AU - Freeke van de Voort
PY - 2026
DA - 2026/10/01
JO - Monthly Notices of the Royal Astronomical Society
DO - 10.1093/mnras/stag1871
UR - https://academic.oup.com/mnras/article/doi/10.1093/mnras/stag1871/8859316
AB - Radio continuum observations are widely used to study cosmic ray (CR) electron populations and transport processes in star-forming galaxies, but their interpretation relies on several simplifying assumptions. Here, we revisit three common assumptions: that some vertical radio profiles can be explained by CR advection alone, that radio spectra directly trace the galaxy-wide CR electron spectrum, and that bremsstrahlung and Coulomb losses are negligible for radio-emitting electrons. We model radio emission using time-dependent CR electron evolution in a magnetohydrodynamical simulation of an isolated Milky Way-mass galaxy. CR electron spectra are evolved self-consistently along Lagrangian tracer particles with the Crest framework, including injection at supernova remnants, advection with the gas, and spatially and temporally varying radiative losses. We compare these results to commonly adopted steady-state models. We find that advection-only transport in self-consistently driven galactic winds fails to reproduce the extended vertical radio intensity profiles observed in edge-on galaxies, despite reproducing the observed steepening of spectral indices with height. This is because slowly accelerating winds keep electrons in strong cooling environments for too long. Matching observed radio haloes with advection alone requires unrealistically high mid-plane wind velocities, implying that additional transport or re-acceleration processes are required. Although galaxy-integrated CR electron spectra at radio-emitting energies are similar across models, the resulting synchrotron spectra differ systematically because radio emission is biased towards young electrons in dense, strongly magnetized regions. Finally, we show that bremsstrahlung and Coulomb losses significantly shape radio spectra even when their loss rate is subdominant and therefore cannot be neglected.
SN - 0035-8711
SN - 1365-2966
ER -