The Star Formation History and the spatial distribution of stellar populations in the Ursa Minor Dwarf Spheroidal Galaxy

As a part of a project devoted to the study of the Ursa Minor dSph, the star formation history of the galaxy is presented in this paper. The analysis uses wide field photometry, encompassing about 1deg x 1deg (the total covered area being 0.75 deg^2), which samples the galaxy out to its tidal radius. Derivation of the SFH has been performed using the synthetic partial model technique. The resulting SFH shows that Ursa Minor hosts a predominantly old stellar population, with virtually all the stars formed earlier than 10 Gyr ago and 90% of them formed earlier than 13 Gyr ago. Nevertheless, Ursa Minor color-magnitude diagram shows several stars above the main, old turn-off forming a blue-plume (BP). If these stars were genuine, main-sequence stars, Ursa Minor would have maintained a low star formation rate extending up to 2 Gyr ago. However, several indications (relative amount and spatial distribution of BP stars and difficulty to retain processed gas) play against this possibility. In such context, the most reliable hypothesis is that BP stars are blue-stragglers originating in the old population, Ursa Minor hence remaining the only Milky Way dSph satellite to host a pure old stellar population. A marginally significant age gradient is detected, in the sense that stars in outer regions are slightly younger, in average. The distance of Ursa Minor, has been calculated using the magnitude of the horizontal-branch and a calibration based on Hipparcos data of main sequence sub-dwarfs. We estimated a distance d=76+/-4 Kpc, which is slightly larger than previous estimates. From the RGB color, we estimate a metallicity [Fe/H]=-1.9+/-0.2, in agreement with a previous spectroscopic determination. No metallicity gradients have been detected across the galaxy.