[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.