NOAO >   Observing Info >   Approved Programs >   2002A-0353

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Proposal Information for 2002A-0353


PI: Steven R. Majewski, University of Virginia, srm4n@astsun.astro.virginia.edu
Address: P.O. Box 3818, Charlottesville, VA 22903, US

CoI: Michael H. Siegel, Space Telescope Science Institute
CoI: Sangmo Sohn, University of Virginia
CoI: Ricky J. Patterson, University of Virginia

Title: M/L's in dSph's: Dark Matter or Tidal Disruption?

Abstract: Using a proven photometric technique for separating metal-poor giant stars from foreground dwarfs, we have mapped the distribution of associated giant stars to well past the nominal tidal radius (r_t) of seven Galactic dSph galaxies and globular clusters. In each case, while a King profile suitably fits the central density profiles, the profile departs from a King function at large r, revealing a ``break population" of stars outside the nominal r_t. The relative density at which the profile departs from a King function is correlated with the inferred M/L ratios of these objects. Two hypotheses for this correlation make different predictions for the run of velocity dispersion with radius, \sigma_v(r): (1) Higher M/L objects harbor more extended dark matter halos that support secondary, bound ``break'' populations. This model predicts declining \sigma_v's at large r. (2) Higher M/L ratios reflect higher degrees of virial \it non- equilibrium in the parent objects, thus invalidating a basic precept underlying the use of core \sigma_v's to obtain masses. Tidal disruption predicts \it rising \sigma_v's with r. Our survey identifies exactly the stars needed to undertake this key discriminatory test of dark matter. To wit, we propose to measure \sigma_v(r) trends in the Leo I and Leo II dSphs' to larger radii than have previously been explored. The fact that we have found evidence for tidal tails on each of these most distant satellites of the Milky Way has important implications for the tidal field and total mass of the Galaxy. We are also proposing to extend our photometric survey of these two dSph galaxies in order to cover a large area, probe deeper to improve our S/N, and more clearly delineate any extended or detached tidal tails.


National Optical Astronomy Observatory, 950 North Cherry Avenue, P.O. Box 26732, Tucson, Arizona 85726, Phone: (520) 318-8000, Fax: (520) 318-8360



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