CHANG-ES

XXVI. Insights into cosmic-ray transport from radio halos in edge-on galaxies^⋆

¹ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), 44780 Bochum, Germany
e-mail: mstein@astro.ruhr-uni-bochum.de
² Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁴ CSI – Instituto de Astrofísica de Andalucía, Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁵ University of Arizona, Steward Observatory, 933 N Cherry Ave., Tucson, AZ, 85721 USA
⁶ Ruhr-Universität Bochum, Research Department, Plasmas with Complex Interactions, 44780 Bochum, Germany
⁷ Department of Astronomy, University of Michigan, 311 West Hall, 1085 South University Avenue, Ann Arbor, MI, 48109 USA
⁸ Purple Mountain Observatory, Chinese Academy of Sciences, 10 Yuanhua Road, Nanjing, 210023 PR China
⁹ University of Manitoba, Dept of Physics and Astronomy, Winnipeg, Manitoba, R3T 2N2 Canada
¹⁰ Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland
¹¹ INAF – Istituto di Radioastronomia, Via P. Gobetti 101, 40129 Bologna, Italy
¹² School of Astronomy, Institute for Research in Fundamental Sciences, 19395-5531 Tehran, Iran
¹³ Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany

Received: 29 April 2022
Accepted: 30 September 2022

Abstract

Context. Galactic winds play a key role in regulating the evolution of galaxies over cosmic time. In recent years, the role of cosmic rays (CRs) in the formation of the galactic wind has increasingly gained attention. Therefore, we use radio continuum data to analyse the CR transport in edge-on galaxies.

Aims. With newly reduced radio continuum data of five edge-on galaxies (NGC 891, NGC 3432, NGC 4013, NGC 4157, and NGC 4631), we plan to set new constraints on the morphology of radio halos and the physical properties of galactic winds driven by stellar feedback. By distinguishing between the central and outer regions of the galaxies, our study setup allows us to search for variations in the radio halo profile or CR transport along the galactic disk.

Methods. Data from the LOFAR Two-metre Sky Survey (LoTSS) Data Release 2 at 144 MHz (HBA) and reprocessed data from the Very Large Array (VLA) at 1.5 GHz (L band) from the Continuum Halos in Nearby Galaxies – an EVLA Survey (CHANG-ES) enable us to increase the extent of the analysed radio continuum profile significantly (up to a factor of 2) compared to previous studies. We computed thermal emission maps using a mixture approach with Hα and near-infrared data, which were then subtracted to yield radio synchrotron emission maps. Then we compiled non-thermal spectral index maps and computed intensity profiles using a box integration approach. Lastly, we performed 1D CR transport modelling.

Results. The non-thermal spectral index maps show evidence that the LoTSS maps are affected by thermal absorption in star-forming regions. The scale height analysis reveals that most of the galaxies are equally well fitted with a one-component instead of a two-component exponential profile. We find a bi-modality within our sample. While NGC 3432 and NGC 4013 have similar scale heights in the L band and HBA, the low-frequency scale heights of NGC 891, NGC 4157, and NGC 4631 exceed their high-frequency counterpart significantly. The 1D CR transport modelling shows agreement between the predicted magnetic field strength and the magnetic field strength estimates of equipartition measurements. Additionally, we find an increasing difference in wind velocities (with increasing height over the galactic disk) between the central and outer regions of the analysed galaxies.