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

Abstract

AbstractAt a bulge latitude of b = −4°, the average [Fe/H] and [Mg/H] values are +0.06 and +0.17 dex, roughly 0.2 and 0.7 dex higher than the local thin and thick disk values, respectively, suggesting a large bulge effective yield, perhaps due to 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. At solar [Fe/H], the bulge [Al/Fe] and [α/Fe] ratios are ~ +0.15 dex. Below [Fe/H] < ![CDATA[$ ~ −0.5 dex, the bulge and local thick disk compositions are very similar; but the measured [Mg/Fe], [⟨SiCaTi⟩/Fe], [La/Eu] and dramatic [Cu/Fe] ratios suggest higher SFR in the bulge. However, these composition differences with the thick disk could be due to measurement errors and non-LTE effects.Unusual zig-zag trends of [Cu/Fe] and [Na/Fe] suggest metallicity-dependent nucleosynthesis by core-collapse supernovae in the Type Ia supernova time-delay scenario.The bulge sub-population compositions resemble the local thin and thick disks, but at higher [Fe/H], suggesting a radial [Fe/H] gradient of − $0.04]] > to − 0.05 dex/kpc for both the thin and thick disks. If the bulge formed through accretion of inner thin and thick disk stars, it appears that these stars retained vertical scale heights characteristic of their kinematic origin, resulting in the vertical [Fe/H] gradient and [α/Fe] trends seen today.