CHANG-ES. XIV. Cosmic-ray propagation and magnetic field strengths in the radio halo of NGC 4631

Aims: NGC 4631 is an interacting galaxy that exhibits one of the largest, gaseous halos observed among edge-on galaxies. We aim to examine the synchrotron and cosmic-ray propagation properties of its disk and halo emission with new radio continuum data.
Methods: Radio continuum observations of NGC 4631 were performed with the Karl G. Jansky Very Large Array at C-band (5.99 GHz) in the C and D array configurations, and at L-band (1.57 GHz) in the B, C, and D array configurations. Complementary observations of NGC 4631 with the Effelsberg telescope were performed at 1.42 and 4.85 GHz. The interferometric total intensity data were combined with the single-dish Effelsberg data in order to recover the missing large-scale total power emission. The thermal and nonthermal components of the total radio emission were separated by estimating the thermal contribution through the extinction-corrected Hα emission. The Hα radiation was corrected for extinction using a linear combination of the observed Hα and 24 μm data.
Results: NGC 4631 has a global thermal fraction at 5.99 (1.57) GHz of 14 ± 3% (5.4 ± 1.1%). The mean scale heights of the total emission in the radio halo (thick disk) at 5.99 (1.57) GHz are 1.79 ± 0.54 kpc (1.75 ± 0.27 kpc) and have about the same values for the synchrotron emission. The total magnetic field of NGC 4631 has a mean strength of ⟨B_eq⟩≃9 μG in the disk, and a mean strength of ⟨B_eq⟩≃7 μG in the halo. We also studied a double-lobed background radio galaxy southwest of NGC 4631, which is an FR II radio galaxy according to the distribution of spectral index across the lobes.
Conclusions: From the halo scale heights we estimated that the radio halo is escape-dominated with convective cosmic ray propagation, and conclude that there is a galactic wind in the halo of NGC 4631.

Reduced images are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/632/A10

Based on observations with the 100 m telescope of the Max-Planck-Institut für Radioastronomie (MPIfR) at Effelsberg and the Karl G. Jansky Very Large Array (VLA) operated by the NRAO. The NRAO is a facility of the National Science Foundation operated under agreement by Associated Universities, Inc.