Hot magnetic halo of NGC 628 (M 74)

Abstract

Context. In several spiral galaxies that are observed face-on, large-scale ordered magnetic fields (the so-called magnetic arms) were found. One of the explanations was the action of the magnetic reconnection, which leads to a higher ordering of the magnetic fields. Because it simultaneously converts the energy of the magnetic fields into thermal energy of the surroundings, magnetic reconnection has been considered as a heating mechanism of the interstellar medium for many years. Until recently, no clear observational evidence for this phenomenon was found. Aims. We search for possible signatures of gas heating by magnetic reconnection effects in the radio and X-ray data for the face-on spiral galaxy NGC 628 (M 74), which presents pronounced magnetic arms and evidence for vertical magnetic fields. Methods. The strengths and energy densities of the magnetic field in the spiral and magnetic arms were derived, as were the temperatures and thermal energy densities of the hot gas, for the disk and halo emission. Results. In the regions of magnetic arms, higher order and lower energy density of the magnetic field is found than in the stellar spiral arms. The global temperature of the hot gas is roughly constant throughout the disk. Conclusions. The comparison of the findings with those obtained for the starburst galaxy M 83 suggests that magnetic reconnection heating may be present in the halo of NGC 628. The joint analysis of the properties of the magnetic fields and the hot gas in NGC 628 also provided clues for possible tidal interaction with the companion galaxy.