The Chemical Composition of the Galactic Bulge and Implications for its Evolution

Abstract

The average bulge [Fe/H] and [Mg/H] are +0.06 and +0.17 dex, respectively, in Baade's Window, roughly 0.2 dex higher than the thin disk and ~0.7 dex higher than the local thick disk metallicity. This suggests a higher effective yield in the bulge, perhaps due to more efficient retention of supernova ejecta. The bulge vertical [Fe/H] gradient, at ~0.5 dex/kpc, appears to be due to a changing mixture of sub-populations (near +0.3 dex and -0.3 dex and one possibly near -0.7 dex) with latitude. The bulge is enhanced in O, Mg, Si, Ca, Ti, and Al relative to the sun, with [alpha/Fe]=+0.15 dex at [Fe/H]=0.0 dex. Below [Fe/H]~-0.5 dex, the bulge and local thick disk compositions are very similar, but small [Mg/Fe] and possibly [<SiCaTi>/Fe] enhancements, low [La/Eu] ratios and large [Cu/Fe], relative to the thick disk suggest slightly higher SFR in the bulge. However, these composition differences could simply be due to measurement errors and non-LTE effects. Unfortunately, comparison with the thick disk near solar [Fe/H] suffers considerable confusion, due to poor identification of the local thick disk. Unusual zig-zag abundance trends of [Cu/Fe] and [Na/Fe] suggest nucleosynthesis dominated by core-collapse supernovae with metallicity-dependent yields, in the Type~Ia supernova time-delay scenario. Thus, the bulge sub-population compositions resemble the local thin and thick disks, but at higher [Fe/H], suggesting a radial [Fe/H] gradient in both the thin and thick disks, within the solar circle, near -0.04 to -0.05 dex/kpc. If the bulge was built through accretion of inner disk stars by a bar, it appears that the inner thin and thick disk stars, incorporated into the bulge, retained vertical scale heights characteristic of their kinematic origin, and resulting in the vertical [Fe/H] gradient and [alpha/Fe] trends seen today.

40 pages, 21 figures. This review paper submitted to PASA. Version 2 updates Figure 10 and comments to it. final version: included various missing bibliographic entries

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