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

Two red clumps (RCs) observed in the color-magnitude diagram of the Milky Way bulge are widely accepted as evidence for a giant X-shaped structure originated from the bar instability. We suggested, however, a drastically different interpretation based on the He-enhanced multiple stellar population phenomenon as is observed in globular clusters (GCs). 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. Because CN-strong stars trace a population with enhanced N, Na, and He abundances that 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 as supported by recent cosmological simulations. Our result also calls for the major revision of the 3D structure of the MilkyWay bulge.