Mapping the stellar age of the Milky Way bulge with the VVV^⋆

I. The method

¹ European Southern Observatory, Karl Schwarzschild-Straße 2, 85748 Garching bei München, Germany
e-mail: frsurot@uc.cl
² Instituto de Astrofísica de Canarias, Vía Láctea S/N, 8200 La Laguna, Tenerife, Spain
³ Department of Astrophysics, University of La Laguna, 38200 La Laguna, Tenerife, Spain
⁴ Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna, 4860 Santiago, Chile
⁵ Millennium Institute of Astrophysics, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
⁶ Excellence Cluster Universe, Boltzmann-Straße 2, 85748 Garching bei München, Germany
⁷ Departamento de Ciencias Físicas, Universidad Andrés Bello, República 220, Santiago, Chile
⁸ Vatican Observatory, 00120 Vatican City, Italy
⁹ UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁰ INAF – Astronomical Observatory of Abruzzo, Via M. Maggini sn, 64100 Teramo, Italy
¹¹ INFN, Sezione di Pisa, Largo Pontecorvo 3, 56127 Pisa, Italy
¹² INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹³ Max-Planck-Institut für Astrophysik, Karl Schwarzschild-Straße 1, 85748 Garching bei München, Germany

Received: 1 June 2018
Accepted: 1 February 2019

Abstract

Context. Recent observational programs are providing a global view of the Milky Way bulge that serves as a template for detailed comparison with models and extragalactic bulges. A number of surveys (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.

Aims. 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°.

Methods. 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 ∼2 mag below the main sequence turnoff. To address the contamination by foreground disk stars we adopt a statistical approach by using control-disk fields located at different latitudes (spanning approximately the bulge’s range) and longitudes −30° and +20°. We generate synthetic photometric catalogs of complex stellar populations with different age and metallicity distributions, including the observational errors and completeness. The comparison between the color–magnitude diagrams of synthetic and observed disk-decontaminated bulge populations provides constraints on the stellar ages in the observed field.

Results. We find the bulk of the bulge stellar population in the observed field along the minor axis to be older than ∼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 ∼7.5 Gyr and 11 Gyr. However, the fraction of stars younger than ∼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 and young (≲5 Gyr) stars.