11252( 1) - 03/30/07 11:08 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 11252 Version: 1 Check-in Time: 30-Mar-2007 15:08:25 STScI Edit Number: 0 Title Ultraluminous X-ray sources in elliptical galaxies and the X-ray binary/globular cluster connection ------------------------------------------------------------------------------------ Type Cycle Parallel Pointing Tolerance AR 16 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Elena Gallo University of California - Santa Barbara CoI: Prof. Tommaso L. Treu University of California - Santa Barbara Y CoI: Prof. Lars Bildsten University of California - Santa Barbara N ------------------------------------------------------------------------------------ Abstract We propose to exploit archival HST/ACS observations of 100 spheroidal galaxies in the Virgo cluster in order to identify the optical counterparts of about 4500 bright X-ray binaries from our ongoing Chandra survey. Based on the shape of the cumulative luminosity function scaled to the mass of the whole sample, we expect to detect around 100 Ultraluminous X-ray Sources (ULXs): the combination of the high resolution and sensitivity of the ACS images will effectively allow us, for the first time, to discriminate between background sources and genuine ULXs for each and every candidate. This, in turn, will allow us to determine whether early-type galaxies harbor ULXs in abundance, and, if so, whether they are preferably in globular clusters, also readily identifiable from the HST images. In addition, this unprecedented catalog of optical counterparts of X-ray sources will yield new information on the properties of low mass X-ray binaries and their association with globular clusters. Finally, by producing a large and clean sample, our study will settle the debate on the presence of a break in the X-ray luminosity function of this population. ------------------------------------------------------------------------------------