D ec 1 99 8 Evolutionary Histories of Dwarf Galaxies in the Local Group

The star formation histories of Local Group (LG) dwarf galaxies and more distant potential LG members are reviewed. Problems in defining the spatial extent of the LG and membership are briefly discussed. The morphological types found in the LG are presented, and it is suggested that we see continuous evolution from low-mass dwarf irregulars (dIrrs) to dwarf spheroidal galaxies (dSph) in the LG. Star formation histories for LG dwarfs and nearby LG candidates are compiled using population boxes. No two dwarfs, irrespective of morphological type, show the same evolutionary history, and all vary widely in ages of their subpopulations and in their enrichment history. The lack of gas in dSphs and certain dwarf ellipticals (dEs) is puzzling both with respect to their star formation histories and the expected mass loss from red giants, but a new photoionization scenario may reconcile these contradictions. Old populations, often spatially very extended, may be a common property of dwarf galaxies, though their fractions can be very small. Almost all types of dwarf galaxies studied in detail so far show spatial variations in ages and abundances such as radial age/metallicity gradients. The observed star formation histories impose constraints on merger and accretion scenarios. Properties of the Milky Way dwarf spheroidals are compared to the M31 dSphs and discussed in the framework of the ram pressure/tidal stripping scenario. It is demonstrated that the newly discovered LG dwarfs follow the same relationship for central surface brightness, mean metallicity, and absolute magnitude as the other LG dwarfs.