Separating the conjoined red clump in the Galactic bulge : kinematics and abundances

Propris, Roberto De and Rich, R. Michael and Kunder, Andrea and Johnson, Christian I. and Koch, Andreas and Brough, Sarah and Conselice, Christopher J. and Gunawardhana, Madusha and Palamara, David and Pimbblet, Kevin and Wijesinghe, Dinuka (2011) Separating the conjoined red clump in the Galactic bulge : kinematics and abundances. Astrophysical Journal Letters, 732 (2): L36. ISSN 2041-8205

Abstract

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.