The not so simple stellar system Omega Cen. II. Evidence in support of a
  merging scenario
  We present multi-band photometry covering $\sim$ 5\deg $\times$ 5\deg across
$\omega$ Cen collected with the Dark Energy Camera, combined to Hubble Space
Telescope and Wide Field Imager data for the central regions. The unprecedented
photometric accuracy and field coverage allowed us to confirm the different
spatial distribution of blue and red main-sequence stars, and of red-giant
branch (RGB) stars with different metallicities. The ratio of the number of
blue to red main-sequence stars shows that the blue main-sequence
sub-population has a more extended spatial distribution compared to the red
main-sequence one, and the frequency of blue main-sequence stars increases at a
distance of $\sim$ 20 arcmin from $\omega$ Cen center. Similarly, the more
metal-rich RGB stars show a more extended spatial distribution compared to the
more metal-poor ones in the outskirts of the cluster. Moreover, the centers of
the distributions of metal-rich and metal-poor RGB stars are shifted in
different directions with respect to the geometrical center of $\omega$ Cen. We
constructed stellar density profiles for the blue and red main-sequence stars;
they confirm that the blue main-sequence sub-population has a more extended
spatial distribution compared to the red main-sequence one in the outskirts of
$\omega$ Cen, as found based on the star number ratio. We also computed the
ellipticity profile of $\omega$ Cen, which has a maximum value of 0.16 at a
distance of $\sim$ 8 arcmin from the center, and a minimum of 0.05 at $\sim$ 30
arcmin; the average ellipticity is $\sim0.10$. The circumstantial evidence
presented in this work suggests a merging scenario for the formation of the
peculiar stellar system $\omega$ Cen.
