Reddening and metallicity maps of the Milky Way bulge from VVV and 2MASS
  III. The first global photometric metallicity map of the Galactic bulge
  We investigate the large scale metallicity distribution in the Galactic
bulge, using a large spatial coverage, in order to constrain the bulge
formation scenario. We use the VISTA variables in the Via Lactea (VVV) survey
data and 2MASS photometry, covering 320 sqdeg of the Galactic bulge, to derive
photometric metallicities by interpolating of the (J-Ks)0 colors of individual
Red Giant Branch stars based on a set of globular cluster ridge lines. We then
use this information to construct the first global metallicity map of the bulge
with a resolution of 30'x45'. The metallicity map of the bulge revealed a clear
vertical metallicity gradient of ~0.04 dex/deg (~0.28 dex/kpc), with metal-rich
stars ([Fe/H]~0) dominating the inner bulge in regions closer to the galactic
plane (|b|<5). At larger scale heights, the mean metallicity of the bulge
population becomes significantly more metal-poor. This fits in the scenario of
a boxy-bulge originated from the vertical inestability of the Galactic bar,
formed early via secular evolution of a two component stellar disk. Older,
metal-poor stars dominate at higher scale heights due to the non-mixed orbits
from the originally hotter thick disk stars.
