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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.