Astronomy and Astrophysics – Astrophysics
Scientific paper
2006-02-08
Astronomy and Astrophysics
Astrophysics
submitted to ApJ letters, 4 pages emulateapj text plus 5 figures
Scientific paper
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.
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