The Radial Extent of the Double Sub Giant Branch in Ngc 1851 ∗

Recent HST-ACS observations revealed the presence of a double sub giant 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 analyse VLT-FORS V, I images, covering 12.7 by 12.7 arcmin, in the South West quadrant of the cluster, allowing us to follow the extent of the double SGB from ∼1 to ∼13 arcmin from the cluster center. Of the two SGB populations, we show that the peculiar one is the fainter one. The percentage of stars in this sequence is about 45% in the innermost region, but starts decreasing already at ∼ 1.5 arcmin from the center. At ∼ 2.4 arcmin from the cluster center, the fainter SGB completely disappears, while the brighter one is still very well defined. Implications for the proposed scenarios are discussed. Subject headings: globular clusters: individual (NGC 1851) – Hertzsprung-Russell (HR) and C-M diagrams