The star formation and chemical evolution histories of the Ursa Minor dwarf spheroidal galaxy

Abstract We derive the star formation history (SFH) and chemical evolution history (CEH) of the Ursa Minor (UMi) dwarf spheroidal galaxy (dSph). We detect two distinct stellar populations that exist over six times the half-light radius from its center. The results are obtained by applying a newly developed algorithm to the deep and wide-field photometric dataset taken with the Hyper Suprime-Cam on the Subaru Telescope. The algorithm employs a genetic algorithm and simulated annealing to minimize a $\chi ^{2}$ value between the observed color–magnitude diagram (CMD) and synthetic CMD generated from the stellar isochrones. The age and metallicity resolutions are set to 0.5 Gyr and 0.1 dex, respectively. The accuracy assessment with mock galaxies shows that it returns the peaks of metallicity distributions and star formation period within 1 $\sigma$ of the input value in the case of a single population. In tests with two populations, two distinct metallicity peaks are identified without an offset from the input values, indicating the robustness of this algorithm. The two detected populations in the UMi dSph have metallicity peaks of [Fe/H] = $-2.2$ and $-2.5$; the metal-rich population started its star formation about 1 Gyr later than the metal-poor one. The SFH of both metal-rich and metal-poor populations varies with distance from the center of the UMi dSph, without any age gradients. These results suggest that the UMi dSph underwent a complex formation process, contrary to the simple formation history of dwarf galaxies previously thought.