The origin of the split red clump in the Galactic bulge of the Milky Way

TL;DR

This study investigates the split red clump in the Milky Way's bulge, linking it to the bulge's peanut shape and disk origin, supported by observations and N-body models.

Contribution

It demonstrates that the split red clump is a common feature of boxy/peanut bulges formed from disks, with a focus on metallicity dependence.

Findings

Split red clump observed at higher latitudes for [Fe/H] > -0.5

Model reproduces the bimodal distribution and kinematics of the split

Bulge's structure is consistent with formation from a disk with few metal-poor stars

Abstract

Near the minor axis of the Galactic bulge, at latitudes b < -5 degrees, the red giant clump stars are split into two components along the line of sight. We investigate this split using the three fields from the ARGOS survey that lie on the minor axis at (l,b) = (0,-5), (0,-7.5), (0,-10) degrees. The separation is evident for stars with [Fe/H] > -0.5 in the two higher-latitude fields, but not in the field at b = -5 degrees. Stars with [Fe/H] < -0.5 do not show the split. We compare the spatial distribution and kinematics of the clump stars with predictions from an evolutionary N-body model of a bulge that grew from a disk via bar-related instabilities. The density distribution of the peanut-shaped model is depressed near its minor axis. This produces a bimodal distribution of stars along the line of sight through the bulge near its minor axis, very much as seen in our observations. The observed and modelled kinematics of the two groups of stars are also similar. We conclude that the split red clump of the bulge is probably a generic feature of boxy/peanut bulges that grew from disks, and that the disk from which the bulge grew had relatively few stars with [Fe/H] < -0.5