Star Formation Histories of Local Group Dwarf Galaxies

40 galaxies are currently known as, or suspected to be LG members within a radius of 1.8 Mpc. 37 of these galaxies are dwarf galaxies with MB > −18.5 mag. I present a compilation of the star formation histories of dwarf irregulars, dwarf ellipticals, and dwarf spheroidals in the Local Group, and visualize their evolutionary histories through Hodge’s population boxes. The study of these star formation histories is a multi-parameter problem: Ages, metallicities, population fractions, and spatial variations must be determined, which depend crucially on the knowledge of reddening and distance. Even the lowest-mass dwarf spheroidal galaxies can show significant spatial variations in star formation, ages, and metallicities. All Local Group dwarf galaxies appear to contain old (> 10 Gyr) and intermediate-age (1–10 Gyr) populations irrespective of morphological type. No two galaxies are alike. They vary widely in star formation histories, metallicities, fractions, and ages of their subpopulations even within the same morphological type. Star formation has occurred either in distinct episodes or continuously over long periods of time. 1 Importance and uniqueness of the Local Group Our knowledge of galaxy formation and evolution starts with the study and understanding of our nearest extragalactic neighbours, the galaxies in the Local Group (LG). LG members are close enough for determinations of ages and metallicities of their resolved stellar populations, ISM abundance studies, and detailed investigations of their star formation histories. Galaxies in the LG comprise a variety of morphological types and cover a wide range of ages, metallicities, and masses. The LG contains both fairly isolated galaxies as well as galaxies in subgroups, which enables us to study environmental influences. The history of the LG also sheds light on the formation and evolution of our own Milky Way. A thorough understanding of the processes in the LG, where stellar populations can be resolved, is the precondition for understanding distant, unresolvable galaxies.