Magnetic field structure and halo in NGC 4631

Abstract

Context. All of the edge-on spiral galaxies observed so far present a similar magnetic field configuration, which consists of a planeparallel 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.

CHANG-ES. XVII. Hα Imaging of Nearby Edge-on Galaxies, New SFRs, and an Extreme Star Formation Region—Data Release 2

“…The behavior of the magnetic field likely depends on the mechanism of the magnetic dynamo on all scales and possibly perturbations from regions of star formation. However, Mora & Krause (2013) find evidence for a global regularity of the magnetic field in NGC 4631 on scales of several kpc. Since regions of star formation occur on smaller scales, a 'disentanglement' of the small-scale magnetic field and the underlying star formation regions may exist above a certain size scale threshold.…”

Thermal instability of halo gas heated by streaming cosmic rays

“…(Beck 2015). There is also evidence for strong magnetic fields (1 − 10µG) that may extend far out (tens of kpc) into the halo (Mora & Krause 2013;Bernet et al 2013). As a result, it is plausible that there are regions in the halo where η is large (e.g., ∼ 1) and/or β is relatively small (e.g., 10).…”

Evolving galactic dynamos and fits to the reversing rotation measures in the halo of NGC 4631

“…The median and standard deviation for this region is shown in the label on this figure and this region has a median error of 29.1 rad/m 2 . This figure was rotated by 5 • as per the position angle of NGC 4631 Mora & Krause (2013). The black lines on this figure show the major and minor axis of the galaxy, note however that there is some curvature to the major axis of this galaxy.…”