Abundances in Local Group Dwarf Elliptical Galaxies

Abundances in Local Group Dwarf Elliptical Galaxies 

The dEs are deceptively simple stellar systems, with no young stars and apparently kinematically-relaxed stellar populations. Recent high-precision ground-based and HST CCD photometry have demonstrated that this is far from the truth. The dEs display a variety of complex multi-episode star formation histories. For example, Carina shows evidence for several distinct star formation events spread over several Gyr (Smecker-Hane et al. 1994) Sculptor and Fornax appear morphologically similar, but Sculptor appears to formed the bulk of its stars at an earlier time than Fornax (Tolstoy et al. 2001). Only 

Ursa Minor appears to have something like a simple, monometallic stellar populations based on its color-magnitude diagram (Mighell Burke 1999, but see below). 

These star formation histories are of vital interest to understanding the evolution of the dEs, and they have raised some puzzles. Among these are the question of how, on the one hand, the dEs lost their gas, and how, on the other hand, they retained gas to experience multiple episodes of star formation Combining the star formation history with the element enrichment history can, in principle, yield the information needed to understand the evolution of dwarf galaxies and their contribution to enrichment of the IGM. 

The main source of uncertainty here is that the Ca/Fe ratio in these stars may not reflect that of the calibrator globular clusters stars - and the Ca/Fe ratio may actually vary with time within each galaxy Nevertheless, with more data like this for the Local Group dwarfs it should be possible to derive very accurate evolution histories for these galaxies. 

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