The Draco and Ursa minor dwarf spheroidal galaxies: A comparative study

We present (V, I) photometry of two wide (similar or equal to25 x 25 arcmin(2)) fields centered on the low surface brightness dwarf spheroidal galaxies Draco and Ursa Minor. New estimates of the distance to these galaxies are provided [(m-M)(0)(UMi) = 19.41 +/- 0.12 and (m-M)(0)(Dra) = 19.84 +/- 0.14], and a comparative study of their evolved stellar population is presented. We detect for the first time the red giant branch (RGB) bump in the luminosity function of UMi (V-RGB(bump) = 19.40 +/- 0.06), while the feature is not detected in Draco. Photometric metallicity distributions are obtained for the two galaxies, and an accurate analysis to determine the intrinsic metallicity spread is performed by means of artificial star experiments. The adopted method is insensitive to stars more metal poor than [Fe/H] similar to -2.5, and it rests on the assumption that the age spread in the considered populations is small (i.e., the impact of the actual age spread on the colors of the RGB stars is negligible). We find that while the average metallicity of the two galaxies is similar ([[Fe/H]](UMi) = -1.8 and [Fe/H](Dra) = -1.7), the metallicity distributions are significantly different, having different peak values ([Fe/H](UMi)(mod) = -1.9 and [Fe/H](Dra)(mod) = -1.6) and different maximum metallicities. We suggest that such differences may be partly responsible for the difference in horizontal-branch morphology between the two galaxies. The intrinsic metallicity 1 sigma spread is sigma(i) = 0.10 in UMi and sigma(i) = 0.13 in Draco. We demonstrate that the inner region of UMi is significantly structured, at odds with what is expected for a system in dynamical equilibrium. In particular, we show that the main density peak of UMi is sigma(i) = center with respect to the center of symmetry of the whole galaxy and shows a much lower ellipticity with respect to the rest of the galaxy. Moreover, UMi stars are shown to be clustered according to two different characteristic clustering scales, as opposed to Draco, which instead has a very symmetric and smooth density profile. The possible consequences of this striking structural difference on our ideas about galaxy formation are briefly discussed. Combining our distance modulus with the more recent estimates of the total luminosity of UMi, we find that the mass-to-light ratio (M/L) of this galaxy may be as low as M/L similar to 7, a factor of 5-10 lower than current estimates.