  The nearly face-on SBc galaxy M83 (NGC 5236) was observed for 25 ksec with
the ROSAT PSPC. We detected 13 point-like sources in this galaxy, 10 of which
were previously unknown. We measured extended X-ray radiation from almost the
whole optically visible galaxy. Comparing the diffuse soft and hard X-ray
emission components, we observed a different asymmetric distribution and a
slower radial decrease of the intensity profile of the soft X-ray emission.
Both these results support the existence of a huge spherical gas halo of 10-15
kpc radius. On the other hand, the radial scale lengths of the hard X-ray
radiation, that of the thermal radio emission and the profile of the optical
surface brightness are similar, favouring the idea that all these emission
processes are connected to star formation in the galaxy's disk. M83 is the
first face-on galaxy where the diffuse X-ray emission spectrum can be
characterized by a two-temperature thermal plasma: a soft X-ray emitting warm
`halo component' and an internally absorbed hot `disk component' which is
dominating the emission in the hard (0.5-2.0 keV) ROSAT energy range. The
combination of X-ray and radio polarization observations allows an estimate of
the plasma parameter beta = U_therm/U_magn which is found to be 0.2+/-0.1. This
result supports the hypothesis that magnetic fields play an important role for
the evolution and structure of galactic gas haloes. The high energy input rate
in the active star-forming disk of M83 seems to be responsible for the outflow
of hot gas and the halo formation.
