Reddening and metallicity maps of the Milky Way Bulge from VVV and 2MASS. I.The method and minor axis maps

Abstract

We present a method to obtain reddening maps and to trace structure and metallicity gradients of the bulge using data from the recently started ESO public survey Vista Variables in the Via Lactea (VVV). We derive the mean J-Ks 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 are then used to build the bulge luminosity function in regions of 0.4 x 0.4 deg to obtain the mean RC magnitudes. These are used as distance indicator in order to trace the bulge structure. Finally, for each subfield we derive photometric metallicities through interpolation of red giant branch colors on a set of empirical ridge lines. The photometric metallicity distributions are compared to metallicity distributions obtained from high resolution spectroscopy in the same regions. The reddening determination is sensitive to small scale variations which are clearly visible in our maps. The luminosity function clearly shows the double RC recently discovered in 2MASS and OGLE III datasets, hence allowing to trace the X-shape morphology of the bulge. Finally, the mean of the derived photometric metallicity distributions are in remarkable agreement with those obtained from spectroscopy. The VVV survey presents a unique tool to map 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.