Reddening and metallicity maps of the Milky Way bulge from VVV and 2MASS

Abstract

Aims: We investigate the large-scale metallicity distribution in the Galactic bulge using large spatial coverage to constrain the bulge formation scenario. Methods: We use the VISTA variables in the Via Lactea (VVV) survey data and 2MASS photometry, which cover 320 sqdeg of the Galactic bulge, to derive photometric metallicities by interpolating 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' × 45'. Results: 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. Conclusions: This fits in the scenario of a boxy bulge originating from the vertical instability 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 of originally hotter thick disk stars. Based on observations taken within the ESO VISTA Public Survey VVV, Program ID 179.B-2002.