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

## TL;DR

This paper introduces a new method for mapping stellar ages in the Milky Way bulge using deep photometry from the VVV survey, revealing that most bulge stars are older than 7.5 billion years and challenging the presence of significant younger populations.

## Contribution

It presents a novel age-dating methodology for bulge stars using PSF-fitting photometry and synthetic population fitting, providing the first age map of the Galactic bulge.

## Key findings

- Most bulge stars are older than 7.5 Gyr.
- The bulge's color-magnitude diagram is incompatible with a large intermediate-age population.
- The age distribution suggests a predominantly old stellar population in the bulge.

## 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 V\'{i}a L\'{a}ctea (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.