Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009hst..prop12274v&link_type=abstract
HST Proposal ID #12274. Cycle 18
Astronomy and Astrophysics
Astrophysics
Scientific paper
Intermediate Mass Black Holes {IMBHs} are objects of considerable astrophysical significance, and the centers of globular clusters may be our best chance of detecting them. Tentative evidence has been presented for, e.g., G1, Omega Cen, and M54. These are all suspected to be the remnant nuclei of disrupted dwarf galaxies, with M54 residing at the center of the Sagittarius dwarf galaxy. IMBHs in these clusters could indicate an important connection between globular cluster black holes and the super-massive black holes known to exist in galactic nuclei. However, the line-of-sight velocity studies from which the IMBH evidence was obtained are not clear-cut. We have demonstrated that HST proper motion studies can provide much better constraints. We recently published 170,000 proper motions for Omega Cen, and our modeling of the results significantly weakened the case for an IMBH. Here we propose a similar study for M54. Ground-based spectroscopy by one of us found a peak in velocity dispersion which could be fit by either a 10^4 solar mass IMBH or an anisotropic velocity distribution; it also found a surprising offset between the kinematic and star count centers. The proposed proper motions will validate and refine our knowledge of the dispersion peak; they will directly measure the anisotropy through the ratio of radial and tangential proper motions; and they will allow an improved determination of the kinematic center from the proper motion dispersion field. We have supporting ground based data; we have all the necessary software for the analysis in hand; we have already astrometrically analyzed the first epoch archival data; and we need only 2 orbits with WFC3/UVIS. We expect a high quality proper motion catalog of 3000 stars, with 300 in the central 5 arcsec. This will put strong new constraints on the M54 cluster dynamics, structure, and possible presence of an IMBH.;
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