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M.L. Houdashelt, R.F.G. Wyse (JHU), G. Gilmore (IoA)
The Ursa Minor dwarf spheroidal (dSph) galaxy has probably had the simplest star-formation history of any Local Group dwarf galaxy. Its entire stellar population appears to be old (>~10~Gyr) and metal-poor ([Fe/H]~~~--2.2), implying a single burst of star formation at a look-back time similar to that inferred for Galactic globular clusters. However, star clusters contain little dark matter, while dwarf galaxies in general, and the UMi dwarf in particular, have been shown to be dark-matter dominated. Could the dark matter in the UMi dSph galaxy be made up of low-mass stars? If so, the faint end of its luminosity function (LF) would have to differ significantly from the LFs of Galactic globular clusters derived from recent HST observations. Such a difference was not seen in the WFPC2 LFs of M92 and the UMi dSph which extended to masses as low as 0.45~M\odot. Here, we further address this question by presenting even deeper HST STIS photometry of the UMi dSph and comparing its faint LF to that of M15 (NGC~7078), a Galactic globular cluster of very similar age and metallicity.
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