The large scale magnetic field structure of the spiral galaxy NGC 5775

Abstract

Context. The origin of large-scale magnetic fields in spiral galaxies is still a theoretical riddle and better observational constraints are required to make further progress. Aims. In order to better determine the large-scale 3D-structure of magnetic fields in spiral galaxies we present a Faraday rotation analysis of the edge-on spiral galaxy NGC 5775. Methods. Deep radio-continuum observations in total power and linear polarization were performed at 8.46 GHz with the VLA and the 100-m Effelsberg telescope. They were analyzed together with archival 4.86 and 1.49 GHz VLA-data. We thus can derive rotation measures from a comparison of three frequencies and determine the intrinsic magnetic field structure. Results. A very extended halo is detected in NGC 5775, with magnetic field lines forming an X-shaped structure. Close to the galactic disk the magnetic field is plane-parallel. The scaleheights of the radio emission esimated for NGC 5775 are comaprable with other galaxies. The rotation measure distribution varies smoothly on both sides along the major axis from positive to negative values. Conclusions. From the derived distribution of rotation measures and the plane-parallel intrinsic magnetic field orientation along the galactic midplane we conclude that NGC 5775 has an even axisymmetric large-scale magnetic field configuration in the disk as generated by an αΩ-dynamo which is accompanied by a quadrupolar poloidal field. The magnetic field lines of the plane-parallel component are pointing outwards. The observed X-shaped halo magnetic field, however, cannot be explained by the action of the disk's mean-field dynamo alone. It is probably due to the influence of the galactic wind together with the dynamo action.

Magnetic field structure and halo in NGC 4631

“…This is expected as Faraday depolarization effects are usually strongest in this area because the thermal electron density, the LOS, and the magnetic field strength are largest. A depolarization along the midplane has already been observed in several other spiral edge-on galaxies, for example NGC 5775 (Soida et al 2011). Furthermore, if there is a magnetic field parallel to the disk accompanied by strong vertical magnetic fields above and below the disk, we also expect strong beam polarization along the region of the projected transition between both magnetic field components.…”

“…The total magnetic field strength in the disk of NGC 4631, B t ≈ 9 ± 2 μG, is comparable to the total field strength of the star forming edge-on galaxy NGC 5775 (Soida et al 2011), but it is not as strong as the strength in the starburst galaxy NGC 253 (Heesen et al 2009) which also has strong star formation along its disk. However, the average strength of the total magnetic field in the halo, B t ≈ 10 ± 2 μG, is higher than in other the edgeon galaxies studied so far.…”

“…In the halo it is characterized by an overall X-shaped configuration and a strong perpendicular field above the galactic center. The edge-on spiral galaxies NGC 5775 (Soida et al 2011) andNGC 4666 (Soida 2005) also have strong perpendicular components above and below their galactic centers. Dynamo simulations by Hanasz et al (2009) may be able to explain not only this X-shaped morphology but also these vertical fields.…”

“…Where spiral arms are visible, the fields tend to be more ordered in the interarm regions. In all of the edge-on spiral galaxies studied up to now, the large-scale ordered fields in the disk lie plane-parallel along the disk of the galaxy and in the halo they have an X-shaped morphology (Krause 2009), sometimes with nearly vertical field components above and below the central region (e.g., in NGC 5775, Soida et al 2011). The disk-parallel A&A 560, A42 (2013) magnetic field is the expected edge-on projection of the spiral magnetic disk field seen in face-on galaxies.…”

Analytical models of X-shape magnetic fields in galactic halos

“…In the past few years, high-sensitivity polarization observations have also revealed the presence in galactic halos of magnetic fields forming a general X pattern. These so-called X-shape magnetic fields are characterized by a vertical (i.e., perpendicular to the disk plane) component that increases with both galactic radius and height in the four quadrants (Tüllmann et al 2000;Soida 2005; Krause et al 2006;Krause 2009;Heesen et al 2009;Braun et al 2010;Soida et al 2011;Haverkorn & Heesen 2012).…”