Two Red Clumps and the X-Shaped Milky Way Bulge
  From 2MASS infra-red photometry we find two red clump (RC) populations
co-existing in the same fields toward the Galactic bulge at latitudes |b|>5.5
deg., ranging over ~13 degrees in longitude and 20 degrees in latitude. We can
only understand the data if these RC peaks simply reflect two stellar
populations separated by ~2.3 kpc; at (l,b)=(+1,-8) the two RCs are located at
6.5 and 8.8+/-0.2 kpc. The double-peaked RC is inconsistent with a tilted bar
morphology. Most of our fields show the two RCs at roughly constant distance
with longitude, which is also inconsistent with a tilted bar, although an
underlying bar may be present. The stellar densities in the two RCs changes
dramatically with longitude: on the positive longitude side the foreground RC
is dominant, while the background RC dominates negative longitudes. A line
connecting the maxima of the foreground and background populations is tilted to
the line of sight by ~20 +/-4 deg., similar to claims for the tilt of a
Galactic bar. The distance between the two RCs decreases towards the Galactic
plane; seen edge-on the bulge is X-shaped, resembling some extra-galactic
bulges and the results of N-body simulations. The center of this X is
consistent with the distance to the Galactic center, although better agreement
would occur if the bulge is 2-3 Gyr younger than 47 Tuc. Our observations may
be understood if the two RC populations emanate, nearly tangentially, from the
ends of a Galactic bar, each side shaped like a funnel or horn. Alternatively,
the X, or double funnel shape, may continue to the Galactic center. This would
appear peanut/box shaped from the Solar direction, but X-shaped when viewed
tangentially.
