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The dwarf spheroidal companion galaxies to the Milky Way have extremely low densities of luminous matter. This allows them to serve as valuable laboratories for detecting the presence of dark matter. Measurements of stellar velocity dispersions in these galaxies over the last decade coupled with the application of simple dynamical models have yielded mass-to-light ratios. For a number of dwarf spheroidals, these mass-to-light ratios significantly exceed those for a normal old stellar population, such as a globular cluster. For example, the apparent mass-to-light ratios of Draco and Ursa Minor are 50-100, while those of globular clusters fall in the range 1-3. The fact that objects this small appear to contain significant amounts of dark matter has important implications, such as ruling out neutrinos as the dark matter based on phase space density constraints, implying very high dark matter densities that guide our thinking on the primordial density fluctuation spectrum, and advancing galaxy formation schemes in which the dark matter plays the dominant role.
In order to strengthen and extend the existing dynamical studies, larger samples of velocities are needed to reduce the statistical error in the velocity dispersions. If these large samples are distributed over the full extent of the galaxy, they permit determination of the system's rotation and the change in velocity dispersion with radius. This latter quantity yields the first information on how mass is distributed within the system.
This talk will concentrate on the dynamics of the Ursa Minor and Draco dwarf spheroidals, the two Milky Way companions with the strongest evidence for very high mass-to-light ratios. We have measured precise radial velocities for large samples of giants in Ursa Minor and Draco using the KPNO 4-m and the multi-fiber instrument Hydra (with E.\ Olszewski and C.\ Pryor). Our sample consists of about 100 stars in each system, with almost all of the stars measured at least twice over three observing seasons. Our samples are about a factor of three larger than those available previously.
An analysis of the radial velocity data will be presented. First, improved velocity dispersions will be given for Draco and Ursa Minor. The change in velocity dispersion with radius will also be examined, as will the rotation of the systems. Significant rotation about the major axis has been found in Ursa Minor, but no rotation has been detected in Draco. Our data also allow us to constrain the number of stars that exhibit velocity variability and thus would be likely binaries. Finally, improved mass-to-light ratios will be calculated based on the new velocity data.
More kinds of data than velocities, of course, are needed to calculate mass-to-light ratios. Uncertainties in the central surface brightnesses and length scales of the dwarf spheroidals contribute significantly to the uncertainties in their mass-to-light ratios. I will also discuss our endeavors to improve the knowledge of these quantities for Draco and Ursa Minor.
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