Milky Way demographics with the VVV survey III. Evidence for a great dark lane in the 157 million star bulge color-magnitude diagram

The new generation of IR surveys are revealing and quantifying Galactic features that provide an improved 3D interpretation of our own Galaxy. We present an analysis of the global distribution of dust clouds in the bulge using the near-IR photometry of 157 million stars from the VVV survey. We investigate the color-magnitude diagram of the Milky Way bulge, which shows a red giant clump of core He burning stars that is split into two color components, with a mean color difference of (Z - K-s) = 0.55 mag that is equivalent to A(V) = 2.0 mag. We conclude that there is an optically thick dust lane at intermediate latitudes above and below the plane that stretches across several square degrees from l = -10 degrees to l = +10 degrees. We call this feature the great dark lane. Although its exact distance is uncertain, it is located in front of the bulge. The evidence for a large-scale great dark lane within the Galactic bulge is important for constraining models of the barred Milky Way bulge and for comparing our galaxy with external barred galaxies in which these types of features are prominent. We discuss two other potential implications of the great dark lane for microlensing and bulge stellar populations studies.