CHANG-ES

XV. Large-scale magnetic field reversals 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, MB R3T 2N2, Canada
¹³ Department of Astronomy and Steward Observatory, University of Arizona, 933 N Cherry Ave, Tucson, AZ 85719, USA

Received: 27 May 2019
Accepted: 29 July 2019

Abstract

Aims. NGC 4631 is an interacting galaxy which exhibits one of the largest gaseous halos observed among edge-on galaxies. We aim to examine the synchrotron and polarization 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. The Rotation Measure Synthesis algorithm was utilized to derive the polarization properties.

Results. We detected linearly polarized emission at C-band and L-band. The magnetic field in the halo is characterized by strong vertical components above and below the central region of the galaxy. The magnetic field in the disk is only clearly seen in the eastern side of NGC 4631, where it is parallel to the plane of the major axis of the galaxy. We detected for the first time a large-scale, smooth Faraday depth pattern in a halo of an external spiral galaxy, which implies the existence of a regular (coherent) magnetic field. A quasi-periodic pattern in Faraday depth with field reversals was found in the northern halo of the galaxy.

Conclusions. The field reversals in the northern halo of NGC 4631, together with the observed polarization angles, indicate giant magnetic ropes with alternating directions. To our knowledge, this is the first time such reversals are observed in an external galaxy.