Hot gas and magnetic arms of NGC 6946: Indications for reconnection heating?^⋆

¹ Astronomisches Institut der Ruhr-Universität Bochum, Universitätsstrasse 150, 44780 Bochum, Germany
e-mail: markmet@oa.uj.edu.pl
² Obserwatorium Astronomiczne Uniwersytetu Jagiellońskiego, ul. Orla 171, 30-244 Kraków, Poland
³ ESA-ESAC, XMM-Newton Science Operations Centre, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 25 June 2015
Accepted: 15 October 2015

Abstract

Context. The grand-design face-on spiral galaxy NGC 6946 is remarkable because of its high star formation activity, the massive northern spiral arm, and the magnetic arms, which are observed in polarized radio synchrotron emission and are located between the optical arms and possibly are magnetic reconnection regions.

Aims. We used electron densities and temperatures in star-forming (active) and less active regions and compared them to findings from the analysis of the radio data to study the energy budget of NGC 6946. The hot gas above the magnetic arms between the optical arms might suggest gas heating by reconnection. We also study the population of point sources in NGC 6946, including the origin of the puzzling ultra-luminous emission complex MF16.

Methods. X-ray observations of NGC 6946 performed with XMM-Newton were used to study the emission from X-ray point sources and diffuse hot gas, including the magnetic arms and the halo. Spectral fitting of the diffuse X-ray emission allowed us to derive temperatures of the hot gas. With assumptions about the emission volume, this allowed us to estimate gas densities, masses, and cooling times.

Results. To explain the X-ray emission from the spiral arms of NGC 6946 two-temperature plasma models are needed to account for the disk and halo emission. The interarm regions show only one thermal component. We observe that the temperature of the hot gas in and above the magnetic arm regions increases slightly when compared to the average temperatures in the areas in and above the spiral arms. For the southwestern part of the disk, which is depolarized in the radio range by Faraday rotation, we find more efficient mixing of disk and halo gas.

Conclusions. We propose magnetic reconnection in the magnetic arm regions of NGC 6946 as the possible cause of the additional heating of the gas and ordering of the magnetic fields. In the southwestern part of the galactic disk we observed indications of a possible faster outflow of the hot gas. A very hot gas within the MF 16 nebula possibly suggests shock heating by a supernova explosion.