Baryonic Properties of the Darkest Galaxies

The faintest and darkest galaxies that we know of today are the dwarf spheroidal (dSph) galaxies. They appear to be plausible counterparts of cosmologically predicted small subhalos though their numbers do not (yet?) suffice to resolve the substructure crisis. Their mass-to-light ratios may go up to 1000 for the faintest objects, and their total masses are of the order of a few 10⁶ to 10⁷ M_⊙. Though most dSphs are dominated by old populations, they all show extended and presumably slow star formation histories with considerable enrichment. While environment has certainly affected their evolution, as evidenced by the morphology-gas-distance relations, intrinsic properties such as their (initial) baryon content may also have played a major role. The complexity and diversity of their star formation histories is surprising, and there are no obvious evolutionary connections to dwarf irregulars.