Galaxy Mergers: A Search for Chemical Signatures

We have gathered high resolution échelle spectra for more than two dozen high-velocity metal-poor field stars, including BD+80 245, a star previously known to have extremely low [α/Fe] abundances, as well as G4-36, a new low [α/Fe] star with unusually large [Ni/Fe]. In this kinematically selected sample, other chemically anomalous stars have also been uncovered. In addition to deriving the α-element abundances, we have also analysed iron-group and s-process elements. Not only does chemical substructure exist in the halo, but the chemical anomalies are not all the same within all elemental groups. Recent kinematic and photometric evidence depicts an increasingly complicated picture of early galaxy formation, and of stellar debris left behind by an by an unknown number of passages and mergers by satellite systems of the Milky Way. In addition to kinematic similarities, chemical “signatures” may identify these merger/accretion events. A number of very high-velocity metalpoor field stars have been discovered that possess very unusual abundance ratios of α-elements (Mg, Si, Ca and maybe Ti) to iron. The stars discovered to date all have large apogalacticon distances—these stars may have originated within a satellite galaxy or galaxies that experienced a different nucleosynthetic chemical evolution history than the Milky Way and which were later accreted by it. Did the nucleosynthetic histories of the dwarf galaxies produce abundance ratios different from those found in the halo of the Milky Way? Preliminary results of chemical abundances of metal-poor stars in the Sagittarius dwarf spheroidal galaxy (Smecker-Hane et al. 1998) as well as in the Draco, Sextans and Ursa Minor dwarf galaxies (Shetrone et al. 2000), for the most part, seem to be similar to those of Galactic halo stars. In the more metal-rich Sgr stars, however, Smecker-Hane et al. find subsolar elemental ratios indicative of an evolution consistent with enrichment from SNIa with declining SNII enrichment. This