Physics
Scientific paper
Jul 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006hst..prop10841c&link_type=abstract
HST Proposal ID #10841. Cycle 15
Physics
Scientific paper
Establishing the presence or absence of intermediate-mass black holes IMBH in globular clusters is crucial for understanding the evolution of dense stellar systems. Observationally, this search has been hampered by the low number of stars with known velocities in the central few arcseconds. This limits our knowledge of the velocity dispersion in the region where the gravitational influence of any IMBH would be felt. In Cycle 13, we successfully obtained ACS/HRC images of the centers of five carefully chosen Galactic globular clusters GO-10401 for a new proper motion study. Although the science case was approved and the first epoch images obtained, the requested future cycle observations were not granted due to a general policy decision based on the strong uncertainties at the time concerning the immediate future of HST. We have now assessed the quality of the first epoch observations. The HRC resolution reveals many isolated stars in to the very center of each cluster that remained blended or unresolved in previous WFPC2 data. Given a two year baseline, we are confident that we can achieve the proper motion precision required to place strict limits on the presence of an IMBH. Therefore, we request the second-epoch, follow-up observations to GO-10401 in order to measure the proper motions of stars in our target clusters. These velocity measurements will allow us to: i place constraints on the mass of a central black hole in each cluster; ii derive the internal velocity dispersion as a function of cluster radius; iii verify or reject previous reports of cluster rotation; and iv directly measure velocity anisotropy as a function of radius. If no second epoch data are obtained then the observing time already invested in the first epoch will have been wasted.
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