Formation, vertex deviation and age of the Milky Way's bulge: input from
  a cosmological simulation with a late-forming bar
  We present the late-time evolution of m12m, a cosmological simulation of a
Milky Way-like galaxy from the FIRE project. The simulation forms a bar after
redshift z = 0.2. We show that the evolution of the model exhibits behaviours
typical of kinematic fractionation, with a bar weaker in older populations, an
X-shape traced by the younger, metal-rich populations and a prominent X-shape
in the edge-on mean metallicity map. Because of the late formation of the bar
in m12m, stars forming after 10 Gyr (z = 0.34) significantly contaminate the
bulge, at a level higher than is observed at high latitudes in the Milky Way,
implying that its bar cannot have formed as late as in m12m. We also study the
model's vertex deviation of the velocity ellipsoid as a function of stellar
metallicity and age in the equivalent of Baade's Window. The formation of the
bar leads to a non-zero vertex deviation. We find that metal-rich stars have a
large vertex deviation (~ 40 degrees), which becomes negligible for metal-poor
stars, a trend also found in the Milky Way, despite not matching in detail. We
demonstrate that the vertex deviation also varies with stellar age and is large
for stars as old as 9 Gyr, while 13 Gyr old stars have negligible vertex
deviation. When we exclude stars that have been accreted, the vertex deviation
is not significantly changed, demonstrating that the observed variation of
vertex deviation with metallicity is not necessarily due to an accreted
population.
