Our Milky Way as a Pure-Disk Galaxy -- A Challenge for Galaxy Formation
  Bulges are commonly believed to form in the dynamical violence of galaxy
collisions and mergers. Here we model the stellar kinematics of the Bulge
Radial Velocity Assay (BRAVA), and find no sign that the Milky Way contains a
classical bulge formed by scrambling pre-existing disks of stars in major
mergers. Rather, the bulge appears to be a bar, seen somewhat end-on, as hinted
from its asymmetric boxy shape. We construct a simple but realistic N-body
model of the Galaxy that self-consistently develops a bar. The bar immediately
buckles and thickens in the vertical direction. As seen from the Sun, the
result resembles the boxy bulge of our Galaxy. The model fits the BRAVA stellar
kinematic data covering the whole bulge strikingly well with no need for a
merger-made classical bulge. The bar in our best fit model has a half-length of
~ 4kpc and extends 20 degrees from the Sun-Galactic Center line. We use the new
kinematic constraints to show that any classical bulge contribution cannot be
larger than ~ 8% of the disk mass. Thus the Galactic bulge is a part of the
disk and not a separate component made in a prior merger. Giant, pure-disk
galaxies like our own present a major challenge to the standard picture in
which galaxy formation is dominated by hierarchical clustering and galaxy
mergers.
