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

I. The method and minor axis maps

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
e-mail: ogonzale@eso.org; mrejkuba@eso.org; evalenti@eso.org
² 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
³ Vatican Observatory, V 00120 Vatican City State, Italy
⁴ European Southern Observatory, Ave. Alonso de Cordova 3107, Vitacura, Santiago, Chile

Received: 30 June 2011
Accepted: 23 July 2011

Abstract

Context. The Milky Way bulge is the nearest galactic bulge and the best laboratory for studies of stellar populations in spheroids based on individual stellar abundances and kinematics. The observed properties point to a very complex nature, which is hard to extrapolate from a few fields.

Aims. We present a method of obtaining reddening maps and tracing structure and metallicity gradients of the bulge using data from the recently started ESO public survey Vista Variables in the Via Lactea (VVV). The method is used to derive properties of the fields along the minor axis.

Methods. We derived the mean J − K_s color of the red clump (RC) giants in 1835 subfields in the bulge region with −8° < b < −0.4° and 0.2° < l < 1.7°, and compare it to the color of RC stars in Baade’s window, for which we adopt E(B − V) = 0.55. This allows us to derive the reddening map on a small enough scale to minimize the problems arising from differential extinction. The dereddened magnitudes were then used to build the bulge luminosity function in regions of  ~0.4° × 0.4° to obtain the mean RC magnitudes. These were used as distance indicator in order to trace the bulge structure. Finally, for each subfield the derived distance and extinction values were used to obtain photometric metallicities through interpolation of red giant branch colors on a set of empirical ridge lines. The photometric metallicity distributions were compared to metallicity distributions obtained from high-resolution spectroscopy in the same regions.

Results. The reddening determination is sensitive to small-scale variations, which are clearly visible in our maps. Our results agree within the errors with literature values based on different methods, although our maps have much higher resolution and more complete coverage. The luminosity function clearly shows the double RC recently discovered in 2MASS and OGLE III datasets, thereby allowing the X-shape morphology of the bulge to be traced. Finally, the mean of the derived photometric metallicity distributions are in remarkable agreement with those obtained from spectroscopy.

Conclusions. The VVV survey is a unique tool for mapping the bulge properties by means of the consistent method presented here. The remarkable agreement between our results and those presented in literature for the minor axis allows us to safely extend our method to the whole region covered by the survey.