Despite the huge amount of photometric and spectroscopic efforts targetting the Galactic bulge over the past few years , its age distribution remains controversial owing to both the complexity of determining the age of individual stars and the difficult observing conditions .
Taking advantage of the recent release of very deep , proper-motion-cleaned colour–magnitude diagrams ( CMDs ) of four low reddening windows obtained with the Hubble Space Telescope ( HST ) , we used the CMD-fitting technique to calculate the star formation history ( SFH ) of the bulge at -2 \degr > b > -4 \degr along the minor axis .
We find that over 80 percent of the stars formed before 8 Gyr ago , but that a significant fraction of the super-solar metallicity stars are younger than this age .
Considering only the stars that are within reach of the current generation of spectrographs ( i.e . V \la 21 ) , we find that 10 percent of the bulge stars are younger than 5 Gyr , while this fraction rises to 20–25 percent in the metal-rich peak .
The age–metallicity relation is well parametrized by a linear fit implying an enrichment rate of dZ / dt \sim 0.005 { Gyr ^ { -1 } } .
Our metallicity distribution function accurately reproduces that observed by several spectroscopic surveys of Baade ’ s window , with the bulk of stars having metal-content in the range [ Fe/H ] \sim - 0.7 to \sim 0.6 , along with a sparse tail to much lower metallicities .