The age distribution of stars in the Milky Way bulge

TL;DR

This study uses a data-driven approach to analyze the ages and chemical compositions of stars in the Milky Way bulge, revealing an older stellar population with complex spatial and kinematic variations.

Contribution

It applies the Cannon method to large survey data to infer stellar ages and abundances specifically in the Galactic bulge, providing new insights into its formation history.

Findings

Bulge stars are approximately twice as old as those in the solar neighborhood.

The bulge exhibits a larger metallicity dispersion and velocity dispersion near the center.

An X-shaped structure is detected at certain latitudes within the bulge.

Abstract

The age and chemical characteristics of the Galactic bulge link to the formation and evolutionary history of the Galaxy. Data-driven methods and large surveys enable stellar ages and precision chemical abundances to be determined for vast regions of the Milky Way, including the bulge. Here, we use the data-driven approach of The Cannon, to infer the ages and abundances for 125,367 stars in the Milky Way, using spectra from Apache Point Observatory Galaxy Evolution Experiment (APOGEE) DR14. We examine the ages and metallicities of 1654 bulge stars within $R_{\text{GAL}}<3.5$ kpc. We focus on fields with $b<12^\circ$, and out to longitudes of $l<15^\circ$. We see that stars in the bulge are about twice as old ($\tau=8$ Gyrs), on average, compared to those in the solar neighborhood ($\tau=4$ Gyrs), with a larger dispersion in [Fe/H] ($\approx0.38$ compared to 0.23 dex). This age gradient comes primarily from the low-$\alpha$ stars. Looking along the Galactic plane, the very central field in the bulge shows by far the largest dispersion in [Fe/H] ($\sigma_{[Fe/H]}\approx0.4$ dex) and line of sight velocity ($\sigma_{vr}\approx90$ km/s), and simultaneously the smallest dispersion in age. Moving out in longitude, the stars become kinematically colder and less dispersed in [Fe/H], but show a much broader range of ages. We see a signature of the X-shape within the bulge at a latitude of $b=8^\circ$, but not at $b=12^\circ$. Future APOGEE and other survey data, with larger sampling, affords the opportunity to extend our approach and study in more detail, to place stronger constraints on models of the Milky Way.