Mapping the stellar age of the Milky Way bulge with the VVV. I. The method

Abstract

Recent observational programmes are providing a global view of the Milky Way bulge that serves as template for detailed comparison with models and extragalactic bulges. A number of surveys (i.e. VVV, GIBS, GES, ARGOS, BRAVA, APOGEE) are producing comprehensive and detailed extinction, metallicity, kinematics and stellar density maps of the Galactic bulge with unprecedented accuracy. However, the still missing key ingredient is the distribution of stellar ages across the bulge. To overcome this limitation, we aim to age-date the stellar population in several bulge fields with the ultimate goal of deriving an age map of the Bulge. This paper presents the methodology and the first results obtained for a field along the Bulge minor axis, at $b=-6^{\circ }$. We use a new PSF-fitting photometry of the VISTA Variables in the Vía Láctea (VVV) survey data to construct deep color-magnitude diagrams of the bulge stellar population down to $\sim$ 2 mag below the Main Sequence turnoff. We find the bulk of the bulge stellar population in the observed field along the minor axis to be at least older than $\sim$ 7.5 Gyr. In particular, when the metallicity distribution function spectroscopically derived by GIBS is used, the best fit to the data is obtained with a combination of synthetic populations with ages in between $\sim$ 7.5 Gyr and 11 Gyr. However, the fraction of stars younger than $\sim$ 10 Gyr strongly depends upon the number of Blue Straggler Stars present in the bulge. Simulations show that the observed color-magnitude diagram of the bulge in the field along the minor axis is incompatible with the presence of a conspicuous population of intermediate-age/young (i.e. $\lesssim 5$ Gyr) stars.

14 pages, 7 figues, Accepted for publication on A&A

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• Using classical Cepheids to study the far side of the Milky Way disk: I. Spectroscopic classification and the metallicity gradient
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• The Blanco DECam Bulge Survey. I. The Survey Description and Early Results
• Unveiling the nature of 12 new low-luminosity Galactic Globular Cluster Candidates
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• Modeling the chemical enrichment history of the Bulge Fossil Fragment Terzan 5
• The VVV Infrared Variability Catalog (VIVA-I)
• The RR Lyrae Delay-Time Distribution: A Novel Perspective on Models of Old Stellar Populations
• Deep PSF photometric catalog of the VVV survey data
• Blanco DECam Bulge Survey (BDBS) V. Cleaning the foreground populations from Galactic bulge colour-magnitude diagrams using Gaia EDR3
• APOGEE view of the globular cluster NGC 6544
• Metallicity-dependent kinematics and orbits in the Milky Way's nuclear stellar disc
• A hundred new eclipsing binary system candidates studied in a near-infrared window in the VVV survey
• OGLE-2016-BLG-1195Lb: A Sub-Neptune Beyond the Snow Line of an M-dwarf Confirmed by Keck AO
• Evidence of distant spiral arms in the Galactic disk quadrant IV from VVV red clump giants
• A Near-Infrared Extinction and Reddening Map Towards the Galactic Bulge Using UKIRT
• Blanco DECam Bulge Survey (BDBS) IV: Metallicity Distributions and Bulge Structure from 2.6 Million Red Clump Stars