X-ray spectra and large-scale features of two starburst galaxies - NGC 253 and M82

We report the results of the spatial and spectral analysis of the Einstein IPC observations of NGC 253 and M82. We also use the MPC data to further constrain the spectral characteristics of M82. We find that the major axis X-ray surface brightness profile of NGC 253 is steeper than the optical profile, but follows closely the radio continuum distribution. This reinforces earlier suggestions of a close link between X-ray and radio continuum emission in spiral galaxies. Extended X-ray emission with no optical or radio counterpart is found in the northern side of this galaxy. We suggest that this emission might be due to gaseous clouds ejected from the starburst nucleus. We also find that the X-ray halo of M82 extends as far as 9' away from the nucleus along the minor axis. The radial distribution of the X-ray surface brightness of this halo is steep and suggests a density distribution ρ is proportional to r^-2^, typical of free flowing winds. If these nuclei, and in particular M82, can be considered representative of primordial galaxies undergoing their first burst of star formation, the present day intracluster media could be the remnants of ancient starburst winds. The spectral properties of the nuclear regions are different from those of the surrounding emission. They are fitted with large intrinsic absorption columns and possibly lower emission temperatures. The X-ray estimates of the extinction are lower than suggested by the infrared observations of these nuclei, but we cannot exclude that this is due to the poorer angular resolution needed for the X-ray spectral analysis. The emission temperatures suggest that the emission of the newly formed massive stars and that of the shock-heated interstellar medium are responsible for most of the X-ray nuclear flux of M82 detected with the IPC. The MPC data, however, suggests the presence of a harder spectral component, which could be due to binary X-ray sources with large intrinsic absorption cutoffs.