Cosmological Implications of Dwarf Spheroidal Chemical Evolution

The chemical properties of dwarf spheroidals in the local group are shown to be inconsistent with star formation being truncated after the reionization epoch (z∼ 8). Enhanced levels of [Ba/Y] in stars in dwarf spheroidals like Sculptor indicate strong s-process production from low-mass stars whose lifetimes are comparable with the duration of the pre-reionization epoch. The chemical evolution of Sculptor is followed using a model with SNe II and SNe Ia feedback and massand metallicitydependent nucleosynthetic yields for elements from H to Pb. We are unable to reproduce the Ba/Y ratio unless stars formed over an interval long enough for the low-mass stars to pollute the interstellar medium with s-elements. This robust result challenges the suggestion that most of the local group dwarf spheroidals are fossils of reionization and supports the case for large initial dark matter halos. Subject headings: galaxies: abundance ratios — galaxies: dwarf — galaxies: Local Group — galaxies: evolution — stars: abundances — stars: AGB — nucleosynthesis