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

Radial extent of the SGB in NGC 1851