The age distribution of stars in the Milky Way 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] ($\approx 0.38$ 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] (${\sigma }_{[Fe/H]}\approx 0.4$ dex) and line of sight velocity (${\sigma }_{vr}\approx 90$ 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.

Published in ApJ. 17 pages, 23 figures. Updated with minor text revisions and additional citations to match ApJ published version

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• A study of the NGC 1193 and NGC 1798 open clusters using CCD UBV photometric and Gaia EDR3 data
• The Swan: Data-Driven Inference of Stellar Surface Gravities for Cool Stars from Photometric Light Curves
• Populating The Milky Way: Characterising Planet Demographics by Combining Galaxy Formation Simulations and Planet Population Synthesis Models
• 3D Parameter Maps of Red Clump Stars in the Milky Way -- Absolute Magnitudes and Intrinsic Colors
• Understanding the velocity distribution of the Galactic Bulge with APOGEE and Gaia
• The chemical and spatial variations of the bulge's velocity ellipsoids
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• Blanco DECam Bulge Survey (BDBS) IV: Metallicity Distributions and Bulge Structure from 2.6 Million Red Clump Stars
• Probing the millisecond pulsar origin of the $\gamma$-ray excess in the Galactic centre with LISA