Hot gas heating via magnetic arms in spiral galaxies. The case of M 83
  Reconnection heating has been considered as a potential source of the heating
of the interstellar medium. In some galaxies, significant polarised radio
emission has been found between the spiral arms. This emission has a form of
`magnetic arms' that resembles the spiral structure of the galaxy. Reconnection
effects could convert some of the energy of the turbulent magnetic field into
the thermal energy of the surrounding medium, leaving more ordered magnetic
fields, as is observed in the magnetic arms. Sensitive radio and X-ray data for
the grand-design spiral galaxy M 83 are used for a detailed analysis of the
possible interactions of magnetic fields with hot gas, including a search for
signatures of gas heating by magnetic reconnection effects. Magnetic field
strengths and energies derived from the radio emission are compared with the
parameters of the hot gas calculated from the model fits to sensitive X-ray
spectra of the hot gas emission. The available X-ray data allowed us to
distinguish two thermal components in the halo of M 83. We found slightly
higher average temperatures of the hot gas in the interarm regions, which
results in higher energies per particle and is accompanied by a decrease in the
energy density of the magnetic fields. The observed differences in the energy
budget between the spiral arms and the interarm regions suggest that, similar
to the case of another spiral galaxy NGC 6946, we may be observing hints for
gas heating by magnetic reconnection effects in the interarm regions. These
effects, which act more efficiently on the turbulent component of the magnetic
field, are expected to be stronger in the spiral arms. However, with the
present data it is only possible to trace them in the interarm regions, where
the star formation and the resulting turbulence is low.
