Formation Histories of Dwarf Galaxies in the Local Group

We compare the properties of dwarf galaxies in the Local Group with the simulated galaxies formed before reionization in a cosmological simulation of unprecedented spatial and mass resolution,including radiative feedback effects. We find that a subset of the Local Group dwarfs are remarkably similar to the simulated dwarf galaxies in all their properties already before reionization. Based on this similarity, we propose the hypothesis that Local Group dwarfs form in a variety of ways: some of them are true fossils'' of the pre-reionization era, some of them form most of their stars later, after reionization (we call them survivors’’ of the reionization era), and the rest of them form an intermediate group of ``polluted fossils’'. We also identify a simple observational test that is able to falsify our hypothesis.