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

Abstract

We have used the Ultraviolet Visual-Echelle Spectrograph (UVES) on Kueyen (UT2) of the Very Large Telescope to take spectra of 15 individual red giant stars in the centers of four nearby dwarf spheroidal galaxies (dSph's): Sculptor, Fornax, Carina, and Leo I. We measure the abundance variations of numerous elements in these low-mass stars with a range of ages (1–15 Gyr old). This means that we can effectively measure the chemical evolution of these galaxies with time . Our results show a significant spread in metallicity with age, but an overall trend consistent with what might be expected from a closed- (or perhaps leaky-) box chemical evolution scenario over the last 10–15 Gyr. We make comparisons between the properties of stars observed in dSph's and in our Galaxy's disk and halo, as well as globular cluster populations in our Galaxy and in the Large Magellanic Cloud. We also look for the signature of the earliest star formation in the universe, which may have occurred in these small systems. We notice that each of these galaxies show broadly similar abundance patterns for all elements measured. This suggests a fairly uniform progression of chemical evolution with time, despite quite a large range of star formation histories. It seems likely that these galaxies had similar initial conditions, and that they evolve in a similar manner with star formation occurring at a uniformly low rate, even if at different times. With our accurate measurements we find evidence for small variations in abundances, which seem to be correlated to variations in star formation histories between different galaxies. The α-element abundances suggest that dSph chemical evolution has not been affected by very high mass stars (>15–20 M ⊙ ). The abundance patterns we measure for stars in dSph's are significantly different from those typically observed in the disk, bulge, and inner halo of our Galaxy. This means that, as far as we can tell from the (limited) data available to date, it is impossible to construct a significant fraction of our disk, inner halo, or bulge from stars formed in dSph's such as we see today, which subsequently merged into our own. Any merger scenario involving dSph's has to occur in the very early universe while they are still gas-rich, so the majority of mass transfer is gas and few stars.