Deciphering the radio-star formation correlation on kpc scales

IV. Radio halos of highly inclined spiral galaxies in the Virgo cluster

¹ Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
² Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland
³ Hamburg University, Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany

^★ Corresponding author: Bernd.Vollmer@astro.unistra.fr

Received: 22 July 2025
Accepted: 1 October 2025

Abstract

In addition to the radio continuum emission of the thin galactic disk, vertically extended emission is ubiquitous in star-forming disk galaxies. This halo emission can represent an important fraction of the total emission of the galaxy. The cosmic-ray electrons (CRe) that cause the radio continuum emission are produced within the thin disk and are transported into the halo. They might interact with the warm neutral and ionized medium, which is also present in the halo region. We made an attempt to reconstruct the radial properties of radio continuum halos in nearly edge-on galaxies, in which the star formation rate (SFR) distribution can be deprojected and the vertical radio continuum emission is clearly distinct from the disk emission. The deprojected SFR distribution was convolved with a Gaussian kernel to take CRe diffusion within the galactic disk into account, and a vertical profile of the radio continuum emissivity was added to the disk emission. The three-dimensional emission distribution was then projected on the sky and was compared to VLA radio continuum observations at 20 and 6 cm. We found that the halo emission overall contains information on the underlying distribution of the SFR. The majority of our galaxies show flaring radio continuum halos. Except for one galaxy, our Virgo galaxies follow the trend of an increasing effective height with increasing radio continuum size, as reported by the CHANG-ES collaboration. We confirm that radio continuum halos can represent a significant fraction of the total radio continuum emission of a star-forming spiral galaxy. At 20 cm and 6 cm, between 30 and 70% of the total radio continuum emission originates in the halo. We propose a halo classification based on the height ratio and SI between 20 and 6 cm. When we interpret the vertical structures of the large-scale magnetic field within the disk-halo and the halo types as a sign of a galactic outflow or wind, all galaxies except one most probably harbor an advection-dominated halo.