Formation Histories of Dwarf Galaxies in the Local Group

We compare the properties of dwarf galaxies in the Local Group with thesimulated galaxies formed before reionization in a cosmological simulation ofunprecedented spatial and mass resolution,including radiative feedback effects.We find that a subset of the Local Group dwarfs are remarkably similar to thesimulated dwarf galaxies in all their properties already before reionization.Based on this similarity, we propose the hypothesis that Local Group dwarfsform in a variety of ways: some of them are ``true fossils'' of thepre-reionization era, some of them form most of their stars later, afterreionization (we call them ``survivors'' of the reionization era), and the restof them form an intermediate group of ``polluted fossils''. We also identify asimple observational test that is able to falsify our hypothesis.Comment: 14 pages including 12 figures. Accepted for publication in ApJ (20 August 2005, v629 2 issue). Note that Fig.12 (predictions on the number of ultrafaint dSphs) is not included in the accepted ApJ versio