The Bulge Metallicity Distribution from the APOGEE Survey

TL;DR

This study analyzes the metallicity distribution of stars in the Milky Way's bulge using APOGEE data, revealing multiple stellar populations and their spatial distributions, which inform the structure and formation history of the Galactic center.

Contribution

First large homogeneous metallicity study of the inner Milky Way bulge using APOGEE data, identifying multiple stellar populations and their spatial metallicity variations.

Findings

Multiple metallicity components indicating diverse stellar populations.

Presence of a super-solar metallicity component near the midplane.

Significant metal-poor population dominates away from the midplane.

Abstract

The Apache Point Observatory Galactic Evolution Experiment (APOGEE) provides spectroscopic information of regions of the inner Milky Way inaccessible to optical surveys. We present the first large study of the metallicity distribution of the innermost Galactic regions based on homogeneous measurements from the SDSS Data Release 12 for 7545 red giant stars within 4.5 kpc of the Galactic center, with the goal to shed light on the structure and origin of the Galactic bulge. Stellar metallicities are found, through multiple-Gaussian decompositions, to be distributed in several components indicative of the presence of various stellar populations such as the bar, or the thin and the thick disk. A super-solar ([Fe/H]=+0.32) and a solar ([Fe/H]=+0.00) metallicity components, tentatively associated with the thin disk and the Galactic bar, respectively, seem to be the major contributors near the midplane. The solar-metallicity component extends outwards in the midplane but is not observed in the innermost regions. The central regions (within 3 kpc of the Galactic center) reveal, on the other hand, the presence of a significant metal-poor population ([Fe/H] =-0.46), tentatively associated with the thick disk, and which becomes the dominant component far from the midplane ($|Z| > +0.75 kpc). Varying contributions from these different components produce a transition region at +0.5 kpc < |Z| < +1.0 kpc characterized by a significant vertical metallicity gradient.