Young stars in an old bulge: a natural outcome of internal evolution in
  the Milky Way
  The center of our disk galaxy, the Milky Way, is dominated by a
boxy/peanut-shaped bulge. Numerous studies of the bulge based on stellar
photometry have concluded that the bulge stars are exclusively old. The
perceived lack of young stars in the bulge strongly constrains its likely
formation scenarios, providing evidence that the bulge is a unique population
that formed early and separately from the disk. However, recent studies of
individual bulge stars using the microlensing technique have reported that they
span a range of ages, emphasizing that the bulge may not be a monolithic
structure. In this letter we demonstrate that the presence of young stars that
are located predominantly near the plane is expected for a bulge that has
formed from the disk via dynamical instabilities. Using an N-body+SPH
simulation of a disk galaxy forming out of gas cooling inside a dark matter
halo and forming stars, we find a qualitative agreement between our model and
the observations of young metal-rich stars in the bulge. We are also able to
partially resolve the apparent contradiction in the literature between results
that argue for a purely old bulge population and those which show a population
comprised of a range in ages; the key is where to look.
