Magnetic field structure and halo in NGC 4631^⋆

Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: caromora@mpifr-bonn.mpg.de; mkrause@mpifr-bonn.mpg.de

Received: 4 January 2013
Accepted: 24 September 2013

Abstract

Context. All of the edge-on spiral galaxies observed so far present a similar magnetic field configuration, which consists of a plane-parallel field in the disk and an X-shaped field at larger z-distances from the plane of the galaxy. Only NGC 4631 seems to have a different field orientation in its disk. Along the eastern and western halves of the disk of NGC 4631 the magnetic field orientation is parallel to the galactic plane, but in the central region of the disk a vertical field seems to dominate.

Aims. In order to clarify whether NGC 4631 has a unique magnetic field configuration in the central region along its disk, we present high-resolution Faraday-corrected polarization data.

Methods. Radio continuum observations of NGC 4631 at 4.85 GHz were performed with the VLA. In addition, observations were made with the Effelsberg telescope at 4.85 GHz and at 8.35 GHz. These were analyzed together with archival VLA-data at 8.35 GHz. The single-dish and interferometer data were combined to recover the missing zero-spacings.

Results. We determined an integrated total spectral index of α_tot =  −0.78 ± 0.04 and a nonthermal integrated spectral index of α_nth =  −0.87 ± 0.03. The vertical scale heights in NGC 4631 vary significantly in different regions within the galaxy and their mean values at 4.85 GHz are with 2.3 kpc (370 pc) for the thick (thin) disk higher than the mean values found so far in six other edge-on spiral galaxies. This may originate in the tidal interaction of NGC 4631 with its neighbouring galaxies. The rotation measures are characterized by a smooth large-scale distribution. Along the galactic plane the degree of Faraday depolarization is significantly high. We estimated a total magnetic field strength in the disk of NGC 4631 of B_t ≈ 9 ± 2   μG and an ordered field strength of B_ord ≈ 2 ± 1   μG. The total field strengths in the halo are of the order of the total magnetic field strength in the disk, whereas the ordered field strengths in the halo seem to be higher than the value in the disk.

Conclusions. The derived distribution of rotation measures implies that NGC 4631 has a large-scale regular magnetic field configuration. Despite the strong Faraday depolarization along the galactic plane and the strong beam depolarization in the transition zone between the disk and halo, our research strongly indicates that the magnetic field orientation along the central 5–7 kpc of the disk is also plane-parallel. Therefore, we claim that NGC 4631 also has a magnetic field structure plane-parallel along its entire disk, similar to all other edge-on galaxies observed up to now.