Age demographics of the Milky Way disc and bulge

Abstract

ABSTRACT We exploit the extensive Gaia Data Release 2 set of long-period variables to select a sample of O-rich Miras throughout the Milky Way disc and bulge for study. Exploiting the relation between Mira pulsation period and stellar age/chemistry, we slice the stellar density of the Galactic disc and bulge as a function of period. We find that the morphology of both components evolves as a function of stellar age/chemistry with the stellar disc 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 disc. We see evidence of a perturbed disc, with large-scale stellar overdensities visible both in and away from the stellar plane. We find that the bulge is well modelled by a triaxial boxy distribution with an axial ratio of ∼1:0.4:0.3. The oldest of the Miras (∼9–10 Gyr) show little bar-like morphology, while the younger stars appear inclined at a viewing angle of ∼21° 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.