Assembling the Milky Way bulge from globular clusters: Evidence from the double red clump

TL;DR

This study provides evidence that the double red clump in the Milky Way bulge results from multiple stellar populations originating in globular clusters, challenging the traditional X-shaped bulge interpretation.

Contribution

It demonstrates that the double RC is due to multiple populations from disrupted globular clusters, revising the understanding of the bulge's structure.

Findings

Significant difference in CN-band strength between the two RCs.

CN-strong stars indicate a population with enhanced N, Na, and He.

Results support the globular cluster origin of the double RC.

Abstract

The two red clumps (RCs) observed in the color-magnitude diagram of the Milky Way bulge is widely accepted as evidence for an X-shaped structure originated from the bar instability. A drastically different interpretation has been suggested, however, based on the He-enhanced multiple stellar population phenomenon as is observed in globular clusters (GCs). Because these two scenarios imply very different pictures on the formation of the bulge and elliptical galaxies, understanding the origin of the double RC is of crucial importance. Here we report our discovery that the stars in the two RCs show a significant (> 5.3 {\sigma}) difference in CN-band strength, in stark contrast to that expected in the X-shaped bulge scenario. The difference in CN abundance and the population ratio between the two RCs are comparable to those observed in GCs between the first- and later generation stars. Since CN-strong stars trace a population with enhanced N, Na, and He abundances originated in GCs, this is direct evidence that the double RC is due to the multiple population phenomenon, and that a significant population of stars in the Milky Way bulge were assembled from disrupted proto-GCs. Our result also calls for the major revision of the 3D structure of the Milky Way bulge given that the current view is based on the previous interpretation of the double RC phenomenon.