Radial extent of the SGB in NGC 1851

TL;DR

This study maps the radial extent of the double subgiant branch in NGC 1851, revealing the faint SGB population is centrally concentrated and diminishes outward, impacting theories on its origin.

Contribution

First to analyze the spatial distribution of the double SGB in NGC 1851, showing the faint SGB is centrally concentrated and decreases with radius.

Findings

Faint SGB population drops to zero around 2.4 arcmin from the center.

Bright SGB remains prevalent at larger radii.

Implications for star formation and pollution scenarios in the cluster.

Abstract

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.