Resolved magnetic structures in the disk-halo interface of NGC 628^⋆

¹ Jodrell Bank Centre for Astrophysics, Alan Turing Building, School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK
e-mail: david.mulcahy@manchester.ac.uk
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ CSIRO Astronomy and Space Science, 26 Dick Perry Avenue, Kensington, WA 6151, Australia
⁴ Netherlands Institute for Radio Astronomy (ASTRON), Postbus 2, 7990 AA Dwingeloo, The Netherlands
⁵ Kapteyn Astronomical Institute, Postbus 800, 9700 AV Groningen, The Netherlands

Received: 14 October 2016
Accepted: 20 December 2016

Abstract

Context. Magnetic fields are essential to fully understand the interstellar medium and its role in the disk-halo interface of galaxies is still poorly understood. Star formation is known to expel hot gas vertically into the halo and these outflows have important consequences for mean-field dynamo theory in that they can be efficient in removing magnetic helicity.

Aims. We aim to probe the vertical magnetic field and enhance our understanding of the disk-halo interaction of galaxies. Studying a face-on galaxy is essential so that the magnetic field components can be separated in 3D.

Methods. We perform new observations of the nearby face-on spiral galaxy NGC 628 with the Karl G. Jansky Very Large Array (JVLA) at S-band (2.6–3.6 GHz effective bandwidth) and the Effelsberg 100-m telescope at frequencies of 2.6 GHz and 8.35 GHz with a bandwidth of 80 MHz and 1.1 GHz, respectively. Owing to the large bandwidth of the JVLA receiving system, we obtain some of the most sensitive radio continuum images in both total and linearly polarised intensity of any external galaxy observed so far.

Results. The application of rotation measures synthesis to the interferometric polarisation data over this large bandwidth provides high-quality images of Faraday depth and polarisation angle from which we obtained evidence for drivers of magnetic turbulence in the disk-halo connection. Such drivers include a superbubble detected via a significant Faraday depth gradient coinciding with a H I hole. We observe an azimuthal periodic pattern in Faraday depth with a pattern wavelength of 3.7 ± 0.1 kpc, indicating Parker instabilities. The lack of a significant anti-correlation between Faraday depth and magnetic pitch angle indicates that these loops are vertical in nature with little helical twisting, unlike in IC 342. We find that the magnetic pitch angle is systematically larger than the morphological pitch angle of the polarisation arms which gives evidence for the action of a large-scale dynamo where the regular magnetic field is not coupled to the gas flow and obtains a significant radial component. We additionally discover a lone region of ordered magnetic field to the north of the galaxy with a high degree of polarisation and a small pitch angle, a feature that has not been observed in any other galaxy so far and is possibly caused by an asymmetric H I hole.

Conclusions. Until now NGC 628 has been relatively unexplored in radio continuum but with its extended H I disk and lack of active star formation in its central region has produced a wealth of interesting magnetic phenomena. We observe evidence for two drivers of magnetic turbulence in the disk-halo connection of NGC 628, namely, Parker instabilities and superbubbles.