Stellar Chemical Signatures And Hierarchical Galaxy Formation

Abstract

ABRIDGED: To compare the chemistries of stars in the Milky Way dSph galaxies with stars in the Galaxy, we have compiled a large sample of Galactic stellar abundances from the literature. As found in previous studies, the [alpha/Fe] ratios of most stars in the dSph galaxies are generally lower than similar metallicity Galactic stars in this extended sample. Our kinematically selected stars confirm that this is true for all major stellar components of the Galaxy, including the halo, thin disk, and thick disk. There is marginal overlap in the low [alpha/Fe] ratios between dSph stars and Galactic halo stars on extreme retrograde orbits, but this is not supported by other element ratios, like [Ba/Y]. Thus, the full chemical signature of most of the dSph stars are distinct from the main components of the Galaxy. This result rules out continuous merging of low mass galaxies similar to these dSph satellites during the formation of the Galaxy, which is discuss further. The new environments offered by the dSph galaxies also allow us to examine fundamental assumptions related to nucleosynthesis. The metal-poor stars ([Fe/H] < -1.8) in the dSph galaxies have [alpha/Fe] ratio that are consistent with predictions from the alpha-process (alpha-rich freeze out). This may also explain the very low Y abundances, and could suggest a separate r-process site for this light (first peak r-process) element. In stars with higher metallicities ([Fe/H] > -1.8), contributions from the s-process are expected; [Ba/Y] is still much higher in the dSph stars than similar metallicity Galactic stars though, consistent with s-process contributions from only the low metallicity AGB stars in dSph galaxies. Finally, the Na-Ni trend in Galactic halo stars is confirmed, but discuss this in terms of nucleosynthesis, and not the accretion of dSphs.