Star Formation Histories of Local Group Dwarf Galaxies

Abstract

40 galaxies are currently known as, or suspected to be Local Group members within a radius of 1.8 Mpc. 37 of these galaxies are dwarf galaxies with M_B > -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.