Separation of Stellar Populations by an Evolving Bar: Implications for
  the Bulge of the Milky Way
  We present a novel interpretation of the previously puzzling different
behaviours of stellar populations of the Milky Way's bulge. We first show, by
means of pure N-body simulations, that initially co-spatial stellar populations
with different in-plane random motions separate when a bar forms. The radially
cooler populations form a strong bar, and are vertically thin and
peanut-shaped, while the hotter populations form a weaker bar and become a
vertically thicker box. We demonstrate that it is the radial, not the vertical,
velocity dispersion that dominates this evolution. Assuming that early stellar
discs heat rapidly as they form, then both the in-plane and vertical random
motions correlate with stellar age and chemistry, leading to different density
distributions for metal-rich and metal-poor stars. We then use a
high-resolution simulation, in which all stars form out of gas, to demonstrate
that this is what happens. When we apply these results to the Milky Way we show
that a very broad range of observed trends for ages, densities, kinematics and
chemistries, that have been presented as evidence for contradictory paths to
the formation of the bulge, are in fact consistent with a bulge which formed
from a continuum of disc stellar populations which were kinematically separated
by the bar. For the first time we are able to account for the bulge's main
trends via a model in which the bulge formed largely in situ. Since the model
is generic, we also predict the general appearance of stellar population maps
of external edge-on galaxies.
