Formation, vertex deviation and age of the Milky Way's bulge: input from a cosmological simulation with a late-forming bar

TL;DR

This study uses a cosmological simulation to analyze the formation and evolution of the Milky Way's bulge, focusing on bar development, stellar kinematics, and metallicity distribution, revealing insights into the galaxy's structural and dynamical history.

Contribution

It provides new insights into the late formation of the galactic bar, the kinematic behavior of different stellar populations, and the relationship between vertex deviation, age, and metallicity in the Milky Way's bulge.

Findings

Bar forms after redshift z=0.2 in the simulation.

Younger, metal-rich stars trace an X-shape in the bulge.

Vertex deviation varies with metallicity and age, matching some Milky Way trends.

Abstract

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.