Star formation histories of dwarf spheroidal and dwarf elliptical galaxies in the local Universe

Abstract

ABSTRACTWe present the star formation histories (SFHs) of early-type dwarf galaxies, dSphs and dEs, in the local Universe within z  = 0.01. The SFHs of early-type dwarf galaxies are characterized by pre-enriched, metal-poor old stellar populations, absence of moderately old stars that have ages of a few Gyr. There are some differences in the SFHs of dSphs and dEs. In particular, dSphs formed old (≳ 10 Gyr old) metal-poor stars ∼2 times more than dEs. The effects of reionization and feedback from supernova explosions are thought to be strong enough to remove the gas left, which prevent moderately old stellar populations in dSphs. In contrast, the ejected gases are not completely removed from dEs and fall back to ignite burst of star formation at a few Gyr after the first period of violent bursts of star formation, showing a suppression of star formation at lookback time ≈9.6 Gyr. The second peak of star formation at lookback time ≈4.5 Gyr in dEs produces moderately old stellar populations. Distinction between dSphs and dEs is useful to examine the SFHS of the early-type dwarfs since the cumulative SFHs are most closely related to their morphology. The stellar mass plays an important role in the SFHs of the early-type dwarfs as a driver of star formation, especially in galaxies with primordial origin.