Kinematics and Composition of the Galactic Bulge: Recent Progress

TL;DR

This paper reviews recent progress in understanding the Galactic bulge's composition and kinematics, highlighting new findings on its chemical abundances and rotation profile that challenge previous models.

Contribution

It provides new observational data on the bulge's chemical composition and rotation, and compares these with existing dynamical models, revealing discrepancies.

Findings

Bulge shows enhanced alpha element abundances.

Rotation curve flattens at larger longitudes, slower than predicted.

Bulge does not rotate as a solid body, contrary to earlier assumptions.

Abstract

We present recent results from a Keck study of the composition of the Galactic bulge, as well as results from the bulge Bulge Radial Velocity Assay (BRAVA). Culminating a 10 year investigation, Fulbright, McWilliam, & Rich (2006, 2007) solved the problem of deriving the iron abundance in the Galactic bulge, and find enhanced alpha element abundances, consistent with the earlier work of McWilliam & Rich (1994). We also report on a radial velocity survey of {\sl 2MASS}-selected M giant stars in the Galactic bulge, observed with the CTIO 4m Hydra multi-object spectrograph. This program is to test dynamical models of the bulge and to search for and map any dynamically cold substructure in the Galactic bulge. We show initial results on fields at $-10^{\circ} < l <+10^{\circ}$ and $b=-4^{\circ}$. We construct a longitude-velocity plot for the bulge stars and the model data, and find that contrary to previous studies, the bulge does not rotate as a solid body; from $-5^{\circ}<l<+5^{\circ}$ the rotation curve has a slope of $\approx 100 km s^{-1}$ and flattens considerably at greater $l$ and reaches a maximum rotation of $45 {km s^{-1}}$ (heliocentric) or $\sim 70 {km s^{-1}}$ (Galactocentric). This rotation is slower than that predicted by the dynamical model of Zhao (1996).