Evolution of Interstellar Clouds in Local Group Dwarf Spheroidal Galaxies in the Context of Their Star Formation Histories

Abstract

We consider evolution of interstellar clouds in Local Group dwarf spheroidal galaxies (dSph's) in the context of their observed star formation histories. The Local Group dSph's generally experienced initial bursts of star formation in their formation epochs (~15 Gyr ago), when hot gas originating from the supernovae could make the cold interstellar clouds evaporate. We find that the maximum size of evaporating clouds is 10 pc. Thus, clouds larger than 10 pc can survive during the initial star formation. These surviving clouds can contribute to the second star formation to produce intermediate-age (~3-10 Gyr ago) stellar populations. Assuming that collisions between clouds induce star formation and that the timescale of the second star formation is a few Gyr, we estimate the total mass of the clouds. The total mass is about 104 M☉, which is 1-3 orders of magnitude smaller than the typical stellar mass of a present dSph. This implies that the initial star formation is dominant over the second star formation, which is broadly consistent with the observed star formation histories. However, the variety of the dSph's in their star formation histories suggests that the effects of environments on the dSph's may be important.