Separating the conjoined red clump in the Galactic Bulge: Kinematics and Abundances

TL;DR

This study uses spectroscopic data to analyze the kinematics and chemical abundances of stars in the Galactic bulge's red clumps, revealing distinct populations and trends related to metallicity and suggesting a complex bulge formation history.

Contribution

It provides the first detailed kinematic and abundance analysis of two red clump populations in the Galactic bulge, highlighting differences linked to metallicity and possible multiple origins.

Findings

No difference in populations based on radial velocities or Mg$b$-index abundances.

Velocity dispersion decreases with increasing metallicity.

Distinct kinematic behavior for stars with [Fe/H] $<-1$, indicating multiple populations.

Abstract

We have used the AAOMEGA spectrograph to obtain R $\sim 1500$ spectra of 714 stars that are members of two red clumps in the Plaut Window Galactic bulge field $(l,b)=0^{\circ},-8^{\circ}$. We discern no difference between the clump populations based on radial velocities or abundances measured from the Mg$b$ index. The velocity dispersion has a strong trend with Mg$b$-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 define further these trends.