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

III. The first global photometric metallicity map of the Galactic bulge

¹ European Southern Observatory, Casilla 19001, Santiago 19, Chile
e-mail: ogonzale@eso.org
² European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
e-mail: mrejkuba@eso.org; evalenti@eso.org
³ Departamento Astronomía y Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Stgo., Chile
e-mail: mzoccali@astro.puc.cl; dante@astro.puc.cl
⁴ The Milky Way Milennium Nucleus, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
⁵ Vatican Observatory, V 00120 Vatican City State, Italy
⁶ Departamento de Ciencia Fisicas, Universidad Andres Bello, Santiago, Chile

Received: 3 December 2012
Accepted: 30 January 2013

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.