Local Group Dwarf Galaxies and the Contribution of the First Stars to Reionization

The Local Group contains some dwarf galaxies that apparently formed all their stars early, lost their gas to winds or ionization, and survive today as ``fossils'' from the epoch of reionization. This study presents new models of these objects based on the hierarchical chemical evolution framework of Tumlinson (2006). The model accurately reproduces the observed luminosity-metallicity relation of fossil dwarf galaxies with minimal free parameters. When calibrated to this relation, the models show that small dark matter ``minihalos'' formed 2 - 8% of their baryonic mass into stars prior during reionization. By tracking the chemical enrichment of these early halos the models specify that metal-free first stars contributed approximately 5 - 10 % of the ionizing photons generated by these small halos and so did not dominate reionization. Models that allow for larger relative contributions from metal-free stars may not generate enough total ionizing photons for early reionization. As significant star formation in early minihalos is a requirement of many successful models for the IGM reionization, these models can be considered to pass a key test of their validity. It appears we have as much to learn about reionization from the universe at z = 0 as at z = 6.
Comment: submitted to ApJ letters, 4 pages emulateapj text plus 5 figures