CHANG-ES

XXXIV. Magnetic field structure in edge-on galaxies: Characterising large-scale magnetic fields in galactic halos

¹ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), 44780 Bochum, Germany
² Ruhr University Bochum, Faculty of Physics and Astronomy, Theoretische Physik IV, 44780 Bochum, Germany
³ Ruhr Astroparticle and Plasma Physics Center (RAPP Center), Bochum, Germany
⁴ University of Manitoba, Department of Physics and Astronomy, Winnipeg, Manitoba R3T 2N2, Canada
⁵ Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁶ Department of Physics, Engineering Physics & Astronomy, Queen’s University, Kingston, ON K7L 3N6, Canada
⁷ Purple Mountain Observatory, Chinese Academy of Sciences, 10 Yuanhua Road, Nanjing 210023, China
⁸ Department of Astronomy, University of Massachusetts, North Pleasant Street, Amherst, MA 01003-9305, USA

^⋆ Corresponding author; mstein@astro.ruhr-uni-bochum.de

Received: 20 September 2024
Accepted: 7 March 2025

Abstract

Context. Understanding the configuration of galactic magnetic fields is essential for interpreting feedback processes in galaxies. Despite their importance, the exact structure of these fields, particularly in galactic halos, remains unclear. Accurate descriptions are crucial for understanding the interaction between star formation and halo magnetisation.

Aims. By systematically analysing the polarisation patterns in halos of nearby galaxies, we aim to deepen the understanding of the interplay between galactic magnetic fields and star formation processes. Here, we focus on the process of magnetising the galactic halo. Furthermore, we provide an analytical description of the observed X shaped halos.

Methods. Based on C band (6 GHz) radio polarimetry data, we manually classified the polarisation patterns of a sample of nearby late-type edge-on galaxies, by using a newly introduced three-class system: disc-dominated, small-scale, and X shaped. We then fitted X shaped patterns to the polarisation data for galaxies classified as X shaped and explored links between the polarisation patterns and other physical properties of these galaxies.

Results. The classification process shows that 11 out of 18 analysed galaxies with extended polarised halo emission display an X shaped polarisation pattern. Galaxies classified as disc-dominated seem less efficient at forming stars than expected for their stellar mass and rotate faster than galaxies with similarly sized HI discs. X shape modelling reveals that the polarisation patterns are best fitted by a constant-angle model, and we observe a correlation between the X shape opening angle and star formation rate surface density, indicating the interplay between the star formation in the disc and the magnetisation of the galactic halo.

Conclusions. The analysis of polarisation patterns in nearby galaxies reveals that most exhibit an X shaped configuration, indicating a common magnetic field structure in galactic halos. The introduced models capture the X shaped morphology and reveal the link between the X shape’s opening angle and star formation rate surface density.