Hubble Space Telescope observations of the local group dwarf galaxy Leo I

We present deep HST F555W (V) and F814W (I) observations of a central field in the Local Group dwarf spheroidal (dSph) galaxy Leo I. The resulting col or-magnitude diagram (CMD) reaches I similar or equal to 26 and reveals the oldest similar or equal to 10-15 Gyr old turnoffs. Nevertheless, a horizon tal branch is not obvious in the CMD. Given the low metallicity of the gala xy, this likely indicates that the first substantial star formation in the galaxy may have been somehow delayed in Leo I in comparison with the other dSph satellites of the Milky Way. The subgiant region is well and uniformly populated from the oldest turnoffs up to the 1 Gyr old turnoff, indicating that star formation has proceeded in a continuous way, with possible varia tions in intensity but no big gaps between successive bursts, over the gala xy's lifetime. The structure of the red clump of core He-burning stars is c onsistent with the large amount of intermediate-age population inferred fro m the main sequence and the subgiant region. In spite of the lack of gas in Leo I, the CMD clearly shows star formation continuing until 1 Gyr ago and possibly until a few hundred Myr ago in the central part of the galaxy.