Stars behind bars I: The Milky Way's central stellar populations

TL;DR

This study demonstrates that a high-resolution cosmological simulation can accurately reproduce the Milky Way's central bulge properties, including its morphology, kinematics, and age-metallicity relations, providing insights into bulge formation.

Contribution

It is the first to use a fully cosmological hydrodynamical simulation to match the Milky Way's bulge features and develop an observational metric for bulge formation timing.

Findings

Simulation reproduces Milky Way's bulge morphology and kinematics.

The age dependence of the X-shape matches observed metallicity splits.

Estimated bar formation occurred approximately 8 Gyr ago.

Abstract

We show for the first time, that a fully cosmological hydrodynamical simulation can reproduce key properties of the innermost region of the Milky Way. Our high resolution simulation matches the profile and kinematics of the Milky Way's boxy/peanut-shaped bulge, and hence we can use it to reconstruct and understand the bulge assembly. In particular, the age dependence of the X-shape morphology of the simulated bulge parallels the observed metallicity dependent split in the red clump stars of the inner Galaxy. We use this feature to derive an observational metric that allows us to quantify when the bulge formed from the disk. The metric we propose can be employed with upcoming survey data to constrain the age of the Milky Way bar. From the split in stellar counts we estimate the formation of the 4~kpc scale bar in the simulation to have happened $t^{\rm bar}_{\rm form}\sim8^{+2}_{-2}$ Gyr ago, in good agreement with conventional methods to measure bar formation in simulations. We test the prospects for observationally differentiating the stars that belong to the bulge/bar compared to the surrounding disk, and find that the inner disk and bulge are practically indistinguishable in both chemistry and ages.