Milky Way demographics with the VVV survey. I. The 84-million star
  colour-magnitude diagram of the Galactic bulge
  The Milky Way (MW) bulge is a fundamental Galactic component for
understanding the formation and evolution of galaxies, in particular our own.
The ESO Public Survey VISTA Variables in the Via Lactea is a deep near-IR
survey mapping the Galactic bulge and southern plane. Data taken during 2010-11
covered 315 deg2 in the bulge area in the JHKs bands. 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.
Photometric data in the JHKs bands were combined to produce a single and huge
data set containing 173.1M+ sources in the three bands. Selecting only the data
points flagged as stellar, the total number of sources is 84.0M+. We built the
largest CMDs 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 centre. The red clump (RC) giants are seen double in magnitude at b ~
-8-10 deg, while in the inner part (b ~ 3deg) they appear to be spreading in
colour, or even splitting into a secondary peak. The analysis of the outermost
bulge area reveals a well-defined sequence of late K and M dwarfs, seen at
(J-Ks) ~ 0.7-0.9 mag and Ks~14 mag. 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 the
outermost bulge, which is important for the planetary transit 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 are caused by
reddening effects.
