The Carina project. II. Stellar populations

We present a new (V, B-V) color-magnitude diagram of the Carina dwarf spheroidal galaxy (dSph) that extends from the tip of the red giant branch (RGB) down to V similar to 25 mag. Data were collected with the Wide Field Imager available at the 2.2 m ESO/MPI telescope and cover an area of approximate to0.3 deg(2) around the center of the galaxy. We confirm the occurrence of a substantial number of old stars with ages around 11 Gyr, together with an intermediate-age population around 5 Gyr. Moreover, we also detected a new, well-defined blue plume of young main-sequence stars with an age, at most, on the order of 1 Gyr. This finding is further supported by the detection of a sizable sample of anomalous Cepheids, whose occurrence can be understood in terms of stars with ages approximate to0.6 Gyr. The evidence for such a young population appears at odds with current cosmological models, which predict that the most recent star formation episodes in dSph's should have taken place 2-3 Gyr ago. At odds with previous results available in the literature, we found that stars along the RGB of old and intermediate-age stellar populations indicate a mean metallicity roughly equal to Z = 0.0004 ([Fe/H] approximate to -1.7) and a small dispersion around this value. This finding is further strengthened by the reduced spread in luminosity of RR Lyrae and horizontal-branch stars in the old stellar population and of the red clump in the intermediate-age group. We find evidence of a smooth spatial distribution of the intermediate-age stellar population (approximate to5 Gyr), which appears more centrally concentrated than the oldest one (approximate to11 Gyr). The radial distribution of the old population appears more clumpy, with a peak off-center by approximate to2' when compared with the Carina center. Star counts show a well-defined shoulder'' in the northeast direction along both the minor and major axes. Current data do not allow us to assess whether this feature is the break in the slope of star-count profiles predicted by Johnston, Sigurdsson, Hernquist.