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M. D. Gregg, A. M. Karick (U.C. Davis/LLNL)
The brightest globular cluster in the halo of M31, cluster G1, has properties which suggest that it is not an ordinary globular but an ultra-compact dwarf galaxy: its velocity dispersion, M/L, and ellipticity are all atypically large, and its color-magnitude diagram suggests an abundance spread. Using the Keck Laser Guide Star Adaptive Optics system with NIRC2, we have begun an imaging campaign of globular clusters in M31 to measure their core sizes. Combining these data with high dispersion spectroscopy will produce masses and M/L ratios to determine if there are additional UCDs masquerading as M31 globulars. UCDs are thought to be the remnant nuclei from tidally stripped dwarf ellipticals or small spirals; finding additional examples in the cluster system of M31 has implications for galaxy formation processes.
The K-band image quality during our first LGS run was very stable over many hours, with Strehl ratios of 0.35 or better, producing point sources with FWHM of 0\farcs05. The core sizes of the clusters, which range from 0\farcs2 to 0\farcs8 can be easily measured from these data. The observing conditions were nearly as good in the J-band, and we obtained both colors for a number of clusters. We discuss our efforts to produce photometrically-calibrated color-magnitude diagrams of the clusters.
This work is supported by National Science Foundation Grant No.~0407445 and was done at the Institute of Geophysics and Planetary Physics, under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract No.~W-7405-Eng-48.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.