The not so simple globular cluster $\omega$ Cen. I. Spatial distribution
  of the multiple stellar populations
  We present a multi-band photometric catalog of $\approx$ 1.7 million cluster
members for a field of view of $\approx$ 2x2 degree across $\omega$ Cen.
Photometry is based on images collected with the Dark Energy Camera on the 4m
Blanco telescope and the Advanced Camera for Surveys on the Hubble Space
Telescope. The unprecedented photometric accuracy and field coverage allowed us
for the first time to investigate the spatial distribution of $\omega$ Cen
multiple populations from the core to the tidal radius, confirming its very
complex structure. We found that the frequency of blue main-sequence stars is
increasing compared to red main-sequence stars starting from a distance of
$\approx$ 25' from the cluster center. Blue main-sequence stars also show a
clumpy spatial distribution, with an excess in the North-East quadrant of the
cluster pointing towards the direction of the Galactic center. Stars belonging
to the reddest and faintest red-giant branch also show a more extended spatial
distribution in the outskirts of $\omega$ Cen, a region never explored before.
Both these stellar sub-populations, according to spectroscopic measurements,
are more metal-rich compared to the cluster main stellar population. These
findings, once confirmed, make $\omega$ Cen the only stellar system currently
known where metal-rich stars have a more extended spatial distribution compared
to metal-poor stars. Kinematic and chemical abundance measurements are now
needed for stars in the external regions of $\omega$ Cen to better characterize
the properties of these sub-populations.
