The Draco and Ursa Minor dwarf spheroidals. A comparative study

Abstract

We present (V,I) photometry of two wide (~25 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 \pm 0.12 and (m-M)_0(Dra)=19.84 \pm 0.14) and a comparative study of their evolved stellar population is presented. We detect for the first time the RGB-bump in the Luminosity Function of UMi (V_{RGB}^{Bump}=19.40\pm 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 stars experiments. (Abridged) We demonstrate that the inner region of UMi is significantly structured, at odds with what expected for a system in dynamical equilibrium. In particular we show that the main density peak of UMi is off-centered with respect to the center of symmetry of the whole galaxy and it 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 opposite 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 (M/L) ratio of this galaxy may be as low as M/L ~ 7, a factor 5-10 lower than current estimates.