On the age of the Milky Way bulge stellar population
The Milky Way (MW) galaxy is not much different from its faraway cousins. However, our position within the MW allows us to study the properties of its stellar populations with exquisite detail in comparison to extragalactic sources. The bulge of the MW (i.e. the stellar population within ~3 kpc from the Galactic center) is the most massive stellar component of the MW hosting very old stars (>10 Gyr), therefore the study of its stellar population properties can shed light on the formation and evolution of the MW as a whole, and of other spiral galaxies at large.
So far, there is a general consensus on the global kinematic, chemical and structural properties of the bulge populations, however the age, or rather, the distribution of the ages of the stars in the bulge is yet to be completely understood.
We aimed at addressing the questions 'How old is the bulge?' and 'Is there a spatial age gradient in the bulge?' through the determination of the stellar ages in the different fields sparsely distributed within a region of 300 deg² centered on the bulge.
We use images from the VISTA Variables in the Vía Láctea (VVV) survey, based in near infrared passbands, to extract accurate magnitude and color of half a billion stars in the bulge area using point spread function fitting.
The newly derived photometric catalogs, used in addition to probe the extinction towards the bulge, will be made publicly available to the entire community.
The contribution of the intervening disk population along the bulge lines of sight has been detected and removed by using a statistical approach in order to obtain a final stars sample that is representative of the bulge population only.
The determination of the stellar ages in different fields is provided through the comparison between the observations and synthetic stellar population models, which have been carefully tailored to account for the observational effects (i.e. distance dispersion, differential reddening, photometric completeness, photometric and systematic uncertainties).
The simulations leading to the construction of synthetic populations have been carried out by using two different methods: i) a model that uses a spectroscopically derived metallicity distribution functions as prior, leaving the age as the only free parameter; ii) a genetic algorithm that finds the best solution within all possible combinations of age and metallicity (i.e. uniform prior in age and metallicity using IAC-POP/Minniac suite).
We ultimately find that the bulge itself appears to be on average old (>9.5 Gyr) throughout its extension (|l| < 10° and -10° < b < +5°), with a mild gradient of about 0.16 Gyr/deg towards the Galactic center.