Chemical evolution of the Galactic bulge as traced by microlensed dwarf
  and subgiant stars. VI. Age and abundance structure of the stellar
  populations in the central sub-kpc of the Milky Way
  We present a detailed elemental abundance study of 90 F and G dwarf, turn-off
and subgiant stars in the Galactic bulge. Based on high-resolution spectra
acquired during gravitational microlensing events, stellar ages and abundances
for 11 elements (Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Zn, Y and Ba) have been
determined. We find that the Galactic bulge has a wide metallicity distribution
with significant peaks at [Fe/H]=-1.09, -0.63, -0.20, +0.12, +0.41. We also
find a high fraction of intermediate-age to young stars: at [Fe/H]>0 more than
35 % are younger than 8 Gyr. For [Fe/H]<-0.5 most stars are 10 Gyr or older. We
have also identified several episodes when significant star formation in the
bulge happened: 3, 6, 8, and 12 Gyr ago. We further find that the "knee" in the
alpha-element abundance trends of the sub-solar metallicity bulge is located at
about 0.1 dex higher [Fe/H] than in the local thick disk. The Galactic bulge
has complex age and abundance properties that appear to be tightly connected to
the main Galactic stellar populations. In particular, the peaks in the
metallicity distribution, the star formation episodes, and the abundance
trends, show similarities with the properties of the Galactic thin and thick
disks. At the same time there are additional components not seen outside the
bulge region, and that most likely can be associated with the Galactic bar. For
instance, the star formation rate appears to have been slightly faster in the
bulge than in the local thick disk, which most likely is an indication of the
denser stellar environment closer to the Galactic centre. Our results
strengthen the observational evidence that support the idea of a secular origin
for the Galactic bulge, formed out of the other main Galactic stellar
populations present in the central regions of our Galaxy.
