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

TL;DR

This study analyzes the chemical composition of the Galactic bulge, revealing its higher metallicity and alpha-element enhancement compared to the disks, and discusses implications for bulge formation and evolution.

Contribution

It provides detailed chemical abundance measurements of the bulge, compares them with disk populations, and interprets these findings in terms of bulge formation scenarios.

Findings

Bulge has higher average metallicity than thin and thick disks.

Alpha-element enhancement suggests rapid star formation in the bulge.

Vertical metallicity gradient due to changing sub-population mixture.

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.