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

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.

The Blanco DECam bulge survey. I. The survey description and early results

“…Although the optical colours we use in BDBS are more sensitive to reddening, they are also more sensitive to age and metallicity; consequently the wide field optical survey of BDBS is an excellent complement to the pencil beam surveys of HST. We believe that the metallicity sensitivity may give us a different perspective on the overall age problem, compared to the infrared age derivation based on VVV (Surot et al 2019).…”

Using classical Cepheids to study the far side of the Milky Way disk

“…The central thick bar, sometimes called the main bar, is seen from an angle of roughly 27 degrees with respect to the Sun-Galactic center line (Wegg & Gerhard 2013). The bulge populations appear to be prevalently old (older than ∼7.5−10 Gyr, see Gennaro et al 2015;Surot et al 2019), but rather metal rich and α-rich (Barbuy et al 2018). On the other hand, the most metalpoor population, as identified by RR Lyrae variables, does not seem to follow the boxy/peanut distribution and kinematics, and might be the faint remnant of a true spheroidal bulge or the densest part of the inner halo (Minniti 1996;Dékány et al 2013Dékány et al , 2018.…”

A hundred new eclipsing binary system candidates studied in a near-infrared window in the VVV survey

“…Black filled circles represent the EB systems studied in this work, while the region below indicates the tile coloured by its star density. Lower panel: NIR CMD from the deep PSF photometry built with a procedure similar to Surot et al (2019). The EBs are depicted with black filled circles, while the Hess diagram for the underlying population of the whole d040 tile is shown in colours.…”