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