The long bar as seen by the VVV survey I. Colour-magnitude diagrams

Context. The VISTA Variable Survey (VVV) is able to map the Galaxy at l <= 0 degrees. with an unpaired depth (at least 3 mag deeper than 2MASS), opening new possibilities for studying the inner structure of the Milky Way. Aims. In this paper we concentrate on the exploitation of these data to better understand the spatial disposition and distribution of the structures present in the inner Milky Way, particularly the Long Bar and its interaction with the inner disc. Methods. To attain this, we present the Ks vs. J - Ks diagrams obtained with VVV of regions with (-20 degrees < l < -8 degrees, vertical bar b vertical bar < 2 degrees). From them we derive the distribution of red clump giants with heliocentric distance as a proxy for the overall stellar structure of the Galaxy. Along with these diagrams, we analysed the distribution of photometrically selected red supergiants, in order to detect events of recent stellar formation. Results. The observations show the presence of a clear overdensity of stars with associated recent stellar formation that we interpret as the traces of the Long Bar, and we derive an angle for it of 41 degrees +/- 5 degrees with the Sun-Galactic centre line, touching the disc near l = 27 degrees and l = -12 degrees. The colour-magnitude diagrams presented here also show a lack of disc stars in several lines of sight, a fact that we associate with the truncation of the disc by the potential of this bar for R-GC <= 5 kpc.