Complexity on Small Scales. II. Metallicities and Ages in the Leo II Dwarf Spheroidal Galaxy

We present metallicities and ages for 52 red giants in the remote Galacticdwarf spheroidal (dSph) galaxy Leo II. These stars cover the entire surfacearea of Leo II and are radial velocity members. We obtained medium-resolutionmulti-fiber spectroscopy with ESO/VLT's FLAMES spectrograph. The metallicitieswere determined based on the near-infrared Ca II triplet. The resultingmetallicity distribution (MD) is asymmetric and peaks at [Fe/H]=-1.74 dex onthe Carretta & Gratton scale. The full range in metallicities extends from -2.4to -1.1 dex. As in other dSphs, no extremely metal-poor red giants were found.We compare Leo II's observed MD with model predictions for several otherGalactic dSphs from the literature. Leo II clearly exhibits a lack of moremetal poor stars, in analogy to the classical G-dwarf problem, which mayindicate a comparable `K-giant problem'. Moreover, its evolution appears tohave been affected by galactic winds. We use our inferred metallicities as aninput parameter for isochrone fits to SDSS photometry and derive approximateages. The resulting age-metallicity distribution covers the full age range from2-15 Gyr on our adopted isochrone scale. During the first 7 Gyr relative to theoldest stars [Fe/H] appears to have remained almost constant. The almostconstant metallicity at higher ages and a slight drop by about 0.3 dexthereafter may be indicative of rejuvenation by low metallicity gas. Overall,the age-metallicity relation appears to support the formation of Leo II frompre-enriched gas. Evidence for enrichment is seen during the recent 2-4 Gyr.Our findings support earlier photometric findings of Leo II as a galaxy with aprominent old and a dominant intermediate-age population. We do not find asignificant radial metallicity gradient nor age gradient in our data.(Abridged)Comment: 23 pages, 12 Figures, accepted for publication in the A