CHANG-ES

XIV. Cosmic-ray propagation and magnetic field strengths in the radio halo of NGC 4631^⋆,⋆⋆

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: silvia.carolina.mora@gmail.com, mkrause@mpifr-bonn.mpg.de
² Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
³ Department of Physics, Engineering Physics, & Astronomy, Queen’s University, Kingston, ON K7L 3N6, Canada
⁴ Observatoire Astronomique de Strasbourg, Université de Strasbourg, CNRS, UMR 7550, 11 Rue de l’Université, 67000 Strasbourg, France
⁵ Astronomisches Institut (AIRUB), Ruhr-Universität Bochum, Universitätsstrasse 150, 44801 Bochum, Germany
⁶ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁷ Department of Astronomy, New Mexico State University, Las Cruces, NM 88001, USA
⁸ Department of Physics and Astronomy, University of New Mexico, 1919 Lomas Blvd. NE, Albuquerque, NM 87131, USA
⁹ CSIRO Astronomy and Space Science, PO Box 1130, Bentley, WA 6102, Australia
¹⁰ Dept. of Astronomy, University of Michigan, 311 West Hall, 1085 S. University Ave., Ann Arbor, MI 48109, USA
¹¹ Dept. of Astronomy, University of Massachusetts, 710 North Pleasant St., Amherst, MA 01003, USA
¹² Department of Physics and Astronomy, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
¹³ Department of Astronomy and Steward Observatory, University of Arizona, 933 N Cherry Ave, Tucson, AZ 85719, USA

Received: 4 November 2018
Accepted: 17 August 2019

Abstract

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.