Magnetic fields and hot gas in M 101^⋆

¹ Obserwatorium Astronomiczne Uniwersytetu Jagiellońskiego, ul. Orla 171, 30-244 Kraków, Poland
e-mail: markmet@oa.uj.edu.pl
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Instytut Astronomii, Wydział Fizyki, Astronomii i Informatyki Stosowanej, Uniwersytet Mikołaja Kopernika, ul. Grudziądzka 5/7, 87-100 Toruń, Poland
⁴ European Space Agency, European Space Astronomy Centre (ESA/ESAC), Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
⁵ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute, 44780 Bochum, Germany

Received: 26 April 2022
Accepted: 3 June 2022

Abstract

Context. Studies of nearby spiral galaxies in radio and X-ray wavelengths reveal the structure and energy balance of the magnetic fields and the hot interstellar medium (ISM). In some spiral galaxies, large-scale ordered magnetic fields have been found between the spiral stellar arms (the so-called magnetic arms). One of the considered explanations of their origin is magnetic reconnection, which according to theoretical studies can efficiently heat the low-density ISM.

Aims. We present, for the first time, high-resolution C-band (5 GHz) radio maps of the nearby face-on spiral galaxy M 101 to study the magnetic fields and verify the existence of the magnetic arms. The analysis of the archival XMM-Newton X-ray data is performed to search for signatures of gas heating by magnetic reconnection effects in the disk and the halo of this galaxy.

Methods. We combine the Very Large Array (VLA) and Effelsberg radio maps of M 101 to restore the large-scale emission lost in the interferometric observations. From the obtained maps, we derive magnetic field strengths and energy densities, and compare them with the properties of the hot gas found with the spectral analysis of the X-ray data.

Results. Most of the X-ray emission likely comes from the hot gas in the halo of M 101. Its temperature is highest above the massive stellar arm and an inter-arm region with enhanced polarised radio emission, as well as in the inter-arm area where neither Hα nor H I emission is visible. In regions outside of the spiral arms lower strengths, energy densities and higher orders of the magnetic fields were observed.

Conclusions. Although M 101 does not possess well-defined magnetic arms, a rudimentary magnetic arm was identified in one of the inter-arm regions. We found weak signatures of additional heating of the ISM there, as well as in the galactic halo, which could be explained by the action of magnetic reconnection.