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A. Ptak (Carnegie Mellon University)
A Chandra guaranteed-time program is surveying an H-alpha selected sample of galaxies, most of which are known to harbor a low-luminosity AGN or starburst. I will report on the detection rate of IXOs in these galaxies and their X-ray color properties. Preliminary results suggest that the detection rate is ~0-1 IXO per galaxy, with more active galaxies tending to have more candidates. Similarly, a survey of HRI data from nearby galaxies also suggests that starburst galaxies tend to have an enhanced number of IXOs. One possibility is that ``top-heavy'' IMFs in starburst galaxies result in progenitor stars sufficiently massive to produce black holes exceeding 10M\odot more often than is the case with normal (e.g., solar-neighborhood) IMFs. The best-studied starburst galaxy with a large number of IXOs is NGC 253, and a detailed analysis of the temporal and spectral properties of its IXOs derived from Chandra, XMM-Newton, ASCA and ROSAT data will be discussed.