The Globular Cluster Origin of the Milky Way Outer Bulge: Evidence from
  Sodium Bimodality
  Recent investigations of the double red clump in the color-magnitude diagram
of the Milky Way bulge cast serious doubts on the structure and formation
origin of the outer bulge. Unlike previous interpretation based on an X-shaped
bulge, stellar evolution models and CN-band observations have suggested that
this feature is another manifestation of the multiple stellar population
phenomenon observed in globular clusters (GCs). This new scenario requires a
significant fraction of the outer bulge stars with chemical patterns uniquely
observed in GCs. Here we show from homogeneous high-quality spectroscopic data
that the red giant branch stars in the outer bulge (> 5.5 degrees from the
Galactic center) are clearly divided into two groups according to Na abundance
in the [Na/Fe] - [Fe/H] plane. The Na-rich stars are also enhanced in Al, while
the differences in O and Mg are not observed between the two Na groups. The
population ratio and the Na and Al differences between the two groups are also
comparable with those observed in metal-rich GCs. The only plausible
explanation for these chemical patterns and characteristics appears to be that
the outer bulge was mostly assembled from disrupted proto-GCs in the early
history of the Milky Way.
