2001ApJ...556...24Z

The ROSAT public archive was used to include a substantial fraction of all Local Group galaxies in a search for low-level active nuclei. Being the nearest galaxies, these objects offer the best spatial resolution and can be examined to the lowest luminosity levels. The ROSAT data for the centers of 18 Local Group members have been analyzed, in 13 cases for the first time. For another six (mostly major) Local Group galaxies, the ROSAT data have been discussed extensively in the literature, and these results are included in the subsequent discussion. Positive or possible detections occur only in four of the major Local Group members, three of which are spiral galaxies. Another two sources in smaller irregular galaxies are close to the formal optical positions of the systems, but it is doubtful that these refer to proper nuclei, and the two sources are considered ``suspicious.'' The Galactic center source Sgr A* still represents the lowest level of nuclear activity within the Local Group. Since the four detections refer to the (five) most optically luminous members of the Local Group, galaxies appear to need at least L_B≳10⁸ L_B,☉_ in order to host a core with some level of activity. The four central X-ray sources are massive black hole candidates. Supporting evidence from data at other wavelengths or from dynamical analyses exists for most of these. There could thus be up to four massive black holes immediately around us. The remaining systems (irregular and smaller spheroidal galaxies) do not seem to contain massive central black holes.