  Recent HST-ACS observations revealed the presence of a double subgiant branch
(SGB) in the core of the Galactic globular cluster NGC 1851. This peculiarity
was tentatively explained by the presence of a second population with either an
age difference of about 1 Gyr, or a higher C+N+O abundance, probably due to
pollution by the first generation of stars.
  In the present Letter, we analyze VLT-FORS V,I images, covering 12.7x12.7
arcmin, in the southwest quadrant of the cluster, allowing us to probe the
extent of the double SGB from ~1.4 to ~13 arcmin from the cluster center. Our
study reveals, for the first time, that the "peculiar" population is the one
associated to the fainter SGB. Indeed, while the percentage of stars in this
sequence is about 45% in the cluster core (as previously found on the basis of
HST-ACS data), we find that it drops sharply, to a level consistent with zero
in our data, at ~2.4 arcmin from the cluster center, where the brighter SGB, in
our sample, still contains ~100 stars. Implications for the proposed scenarios
are discussed.
