VLT/UVES Abundances in Four Nearby Dwarf Spheroidal Galaxies. II. Implications for Understanding Galaxy Evolution

We have used UVES on VLT-UT2 to take spectra of 15 individual red giant starsin the centers of four nearby dwarf spheroidal galaxies: Sculptor, Fornax,Carina and Leo I. We measure the abundance variations of numerous elements inthese low mass stars with a range of ages (1-15Gyr old). This means that we caneffectively measure the chemical evolution of these galaxies WITH TIME. Ourresults show a significant spread in metallicity with age, but an overall trendconsistent with what might be expected from a closed (or perhaps leaky) boxchemical evolution scenario over the last 10-15Gyr. We notice that each ofthese galaxies show broadly similar abundance patterns for all elementsmeasured. This suggests a fairly uniform progression of chemical evolution withtime, despite quite a large range of star formation histories. It seems likelythat these galaxies had similar initial conditions, and evolve in a similarmanner with star formation occurring at a uniformly low rate, even if atdifferent times. With our accurate measurements we find evidence for smallvariations in abundances which are correlated to variations in star formationhistories. The alpha-elements suggest that dSph chemical evolution has not beenaffected by very high mass stars (>15-20 Msun). The abundance patterns wemeasure for stars in dwarf spheroidal galaxies are significantly different fromthose typically observed in the disk, bulge and inner-halo of our Galaxy. Thissuggests that it is NOT possible to construct a significant fraction of ourGalaxy from STARS formed in these dwarf spheroidal galaxies which subsequentlymerged into our own. Any merger scenario involving dSph has to occur in thevery early Universe whilst they are still gas rich, so the majority of masstransfer is gas, and few stars.Comment: 36 pages, 5 tables, 21 figures, accepted by AJ for Feb 200