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

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.

The origin of the structure of large-scale magnetic fields in disc galaxies

“…We have a suitable energy source in the shear itself. We expect this to dissipate field energy through diffusivity and reconnection -here see Weżgowiec et al (2016) who discuss reconnection as a possible source for the heating of the hot gas above the magnetic interarm regions in NGC 6946. 4 We also note that Heald et al (2016) discuss the possibility of a large-scale regular magnetic field in the dischalo interface of M83, and Mulcahy et al (2017) discuss the halo/WIM shear region in NGC 628 as a possible source of magnetic turbulence.…”

Hot gas heating via magnetic arms in spiral galaxies. The case of M 83

“…4.1). Therefore, to compare the properties of the magnetic fields in M 83 with properties of its hot gas (and follow the analysis presented in Weżgowiec et al 2016), we needed to calculate the global properties of the hot gas by combining the information obtained for both the disc and the halo.…”

Role of galactic bars in the formation of spiral arms: A study through orbital and escape dynamics -- I

“…So many giant spirals containing SMBH shows grand design spiral arms (Ann & Lee 2004;Pastorini et al 2007;Seigar et al 2008;Berrier et al 2013;Davis et al 2017). Such grand design spiral arms had been found in M83, NGC 628, NGC 5457, NGC 6946 (Cedrés et al 2013;Frick et al 2016;Weżgowiec et al 2016) etc. Hence bar potential used in model 1 favours the formation of grand design spirals when SMBH is present at the core and the spiral arms emerge from the ends of the bar.…”