The faint optical stellar luminosity function in the Ursa Minor dwarf spheroidal galaxy

Analyses of their internal stellar kinematics imply that the dwarf spheroid al (dSph) companion galaxies to the Milky Way are among the most dark matte r-dominated systems known. Should there be significant dark matter in the f orm of faint stars in these systems, the stellar luminosity function must b e very different from that of a similar metallicity globular cluster, for w hich there is no evidence for dark matter. We present the faint stellar lum inosity function in the Ursa Minor dSph, down to a luminosity corresponding to similar to 0.45 M., derived from new deep Hubble Space Telescope/WFPC2 data. We find a remarkable similarity between this luminosity function, and inferred initial mass function, and those of the globular cluster M92, a c luster of similar age and metallicity to the Ursa Minor dSph.