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 \mbox { $\ > { 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 can not 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 ( \sim 40 ^ { \circ } ) , 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 \mbox { $\ > { Gyr } $ } , while 13 \mbox { $\ > { 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 .