Age Demographics of the Milky Way Disk and Bulge

TL;DR

This study uses Gaia DR2 data on Mira variables to analyze the age-related structural evolution of the Milky Way's disk and bulge, revealing inside-out formation, disk perturbations, and bar development over billions of years.

Contribution

It introduces a novel method of using Mira pulsation periods to trace the Milky Way's structural evolution across different stellar ages.

Findings

The disk is thicker and more radially compact at older ages.

The bulge is well modeled by a triaxial boxy shape with specific axis ratios.

Bar formation occurred 8-9 Gyr ago, with younger Miras showing a peanut/X-shape morphology.

Abstract

We use the extensive $Gaia$ Data Release 2 set of Long Period Variables to select a sample of Oxygen-rich Miras throughout the Milky Way disk and bulge for study. Exploiting the relation between Mira pulsation period and stellar age/chemistry, we slice the stellar density of the Galactic disk and bulge as a function of period. We find the morphology of both components evolves as a function of stellar age/chemistry with the stellar disk being stubby at old ages, becoming progressively thinner and more radially extended at younger stellar ages, consistent with the picture of inside-out and upside-down formation of the Milky Way's disk. We see evidence of a perturbed disk, with large-scale stellar over-densities visible both in and away from the stellar plane. We find the bulge is well modelled by a triaxial boxy distribution with an axis ratio of $\sim [1:0.4:0.3]$. The oldest of the Miras ($\sim$ 9-10 Gyr) show little bar-like morphology, whilst the younger stars appear inclined at a viewing angle of $\sim 21^{\circ}$ to the Sun-Galactic Centre line. This suggests that bar formation and buckling took place 8-9 Gyr ago, with the older Miras being hot enough to avoid being trapped by the growing bar. We find the youngest Miras to exhibit a strong peanut morphology, bearing the characteristic X-shape of an inclined bar structure.