Separating the Conjoined Red Clump in the Galactic Bulge: Kinematics and Abundances

We have used the AAOMEGA spectrograph to obtain R ~ 1500 spectra of 714 stars that are members of two red clumps in the Plaut Window Galactic bulge field (l, b) = (0°, – 8°). We discern no difference between the clump populations based on radial velocities or abundances measured from the Mgb index. The velocity dispersion has a strong trend with Mgb-index metallicity, in the sense of a declining velocity dispersion at higher metallicity. We also find a strong trend in mean radial velocity with abundance. Our red clump sample shows distinctly different kinematics for stars with [Fe/H] <–1, which may plausibly be attributable to a minority classical bulge or inner halo population. The transition between the two groups is smooth. The chemo-dynamical properties of our sample are reminiscent of those of the Milky Way globular cluster system. If correct, this argues for no bulge/halo dichotomy and a relatively rapid star formation history. Large surveys of the composition and kinematics of the bulge clump and red giant branch are needed to further define these trends.