[Normal Galaxies, Clusters of Galaxies -- Oral ]

Extragalactic globular clusters with Chandra

Tom Maccarone, University of Southampton
Arunav Kundu (Michigan State), Steve Zepf (Michigan State)

Globular clusters produce bright (L\_X>10\^36 ergs/sec) X-ray sources with efficiencies, on average, about 100 times as large as those of field stellar populations. Despite this efficiency, the Milky Way still contains fewer than 20 such sources, making statistical studies of which cluster properties are most important for producing X-ray sources difficult. Furthermore, the Milky Way's globular cluster population is relatively homogeneous in metallicity.There are many nearby elliptical galaxies with 10-100 times as many globular clusters as the Milky Way, and with significantly larger numbers of metal rich clusters. Chandra's excellent angular resolution has made it possible both to detect these sources against the strong gas background often found in these galaxies, and to localize these sources well enough to associate them unambiguously with optically detected globular clusters. An important and surprising result of this work is that metal rich globular clusters are significantly more likely to contain X-ray sources than metal poor clusters.The large samples of extragalactic clusters also make it possible to find rare objects. Numerous ultraluminous X-ray sources have been found in extragalactic clusters. Recent results have shown that a growing fraction of these objects can be proved to be variable, indicating that most of the emission must be coming from a single source, providing obseravtional evidence that black holes in globular clusters are far more common than was once thought.