The ACS Survey of Galactic Globular Clusters. III. The Double Subgiant Branch of NGC 1851

TL;DR

This study uses HST ACS photometry to analyze the split subgiant branch of NGC 1851, suggesting multiple stellar populations and comparing it with other globular clusters with similar features.

Contribution

It provides detailed photometric evidence of a double subgiant branch in NGC 1851 and discusses possible origins beyond simple age differences.

Findings

The SGB of NGC 1851 splits into two distinct branches.

Multiple stellar populations are likely present in NGC 1851.

NGC 1851 differs from other clusters with multiple populations in key aspects.

Abstract

Photometry with HST's ACS reveals that the subgiant branch (SGB) of the globular cluster NGC 1851 splits into two well-defined branches. If the split is due only to an age effect, the two SGBs would imply two star formation episodes separated by $\sim$ 1 Gyr. We discuss other anomalies in NGC 1851 which could be interpreted in terms of a double stellar population. Finally, we compare the case of NGC 1851 with the other two globulars known to host multiple stellar populations, and show that all three clusters differ in several important respects.