Milky Way demographics with the VVV survey - I. The 84-million star colour–magnitude diagram of the Galactic bulge

Context. The Milky Way (MW) bulge is a fundamental Galactic componentfor understanding the formation and evolution of galaxies, in particular our own. The ESO Public Survey VISTA Variables in the V´ ia Lactea is a deep near-IR survey mapping the Galactic bulge and southern plane. Particularly for the bulge area, VVV is c overing∼ 315 deg 2 . Data taken during 2010 and 2011 covered the entire bulge area in the JHKs bands. Aims. We used VVV data for the whole bulge area as a single and homogeneous data set to build for the first time a single colour- magnitude diagram (CMD) for the entire Galactic bulge. Methods. Photometric data in the JHKs bands were combined to produce a single and huge data set containing 173, 150, 467 sources in the three bands, for the∼ 315 deg 2 covered by VVV in the bulge. Selecting only the data points fla gged as stellar, the total number of sources is 84, 095, 284. Results. We built the largest colour-magnitude diagrams published up to date, containing 173.1+ million sources for all data points, and more than 84.0 million sources accounting for the stellar sources only. The CMD has a complex shape, mostly owing to the complexity of the stellar population and the effects of extinction and reddening towards the Galactic centr e. The red clump (RC) giants are seen double in magnitude at b∼−8 ◦ − 10 ◦ , while in the inner part (b∼−3 ◦ ) they appear to be spreading in colour, or even splitting into a secondary peak. Stellar population models show the predominance of main-sequence and giant stars. The analysis of the outermost bulge area reveals a well-defined sequence of l ate K and M dwarfs, seen at (J− Ks)∼ 0.7− 0.9 mag and Ks& 14 mag. Conclusions. The interpretation of the CMD yields important information about the MW bulge, showing the fingerprint of its structure and content. We report a well-defined red dwarf sequence in th e outermost bulge, which is important for the planetary tran sit searches of VVV. The double RC in magnitude seen in the outer bulge is the signature of the X-shaped MW bulge, while the spreading of the RC in colour, and even its splitting into a secondary peak, ar e caused by reddening effects. The region around the Galactic centre is harder to interpret because it is strongly affected by reddening and extinction.