2025A&A...699A.243H

Context. Edge-on spiral galaxies give us an outsiders' view of the radio halo that envelops these galaxies. Radio haloes are caused by extra-planar cosmic-ray electrons that emit synchrotron emission in magnetic fields.
Aims. We aim to study the origin of radio haloes around galaxies and infer the role of cosmic rays in supporting the gaseous discs. We test the influence of star formation as the main source of cosmic rays, as well as other fundamental galaxy properties such as mass and size.
Methods. We present a study of radio continuum scale heights in 22 nearby edge-on galaxies from the Continuum HAloes in Nearby Galaxies - an EVLA Survey (CHANG-ES). We employed deep observations with the Jansky Very Large Array in the S-band (2-4 GHz), imaging at 7″ angular resolution. We measured scale heights in three strips within the effective radio continuum radius, and corrected for the influence of angular resolution and inclination angle. We only included galaxies where a distinction between the two disc components can be made in at least one of the strips, which provided us with robust measurements of both scale heights.
Results. We find a strong positive correlation between the scale heights of the thin and thick discs and the star-forming radius, as well as star-formation rate (SFR); moderately strong correlations are found for the mass surface density and the ratio of SFR-to-mass surface density; no correlation is found with SFR surface density alone. Yet the SFR surface density plays a role as well: galaxies with high SFR surface densities have a rather roundish shape, whereas galaxies with little star formation only show a relatively small vertical extent in comparison to their size.
Conclusions. Thick gaseous discs are partially supported by cosmic-ray pressure. Our results are a useful benchmark for simulations of galaxy evolution that include cosmic rays.