Abundances for a large sample of red giants in NGC 1851: hints for a merger of two clusters?

TL;DR

This study analyzes over 120 red giants in NGC 1851, revealing a metallicity spread and evidence supporting the hypothesis that the cluster resulted from the merger of two smaller clusters with distinct chemical properties.

Contribution

It provides the first detailed chemical abundance analysis of a large sample of NGC 1851 stars, supporting the merger scenario of two globular clusters with different metallicities.

Findings

Detected a metallicity spread of 0.06-0.08 dex.

Both star groups show a Na-O anticorrelation.

Metal-poor stars are more centrally concentrated.

Abstract

We present the abundance analysis of a sample of more than 120 red giants in the globular cluster (GC) NGC 1851, based on FLAMES spectra. We find a small but detectable metallicity spread. This spread is compatible with the presence of two different groups of stars with a metallicity difference of 0.06-0.08 dex, in agreement with earlier photometric studies. If stars are divided into these two groups according to their metallicity, both components show a Na-O anticorrelation (signature of a genuine GC nature) of moderate extension. The metal-poor stars are more concentrated than the metal-rich ones. We tentatively propose the hypothesis that NGC 1851 formed from a merger of two individual GCs with a slightly different Fe and alpha-element content, and possibly an age difference up to 1 Gyr. This is supported also by number ratios of stars on the split subgiant and on the bimodal horizontal branches. The distribution of n-capture process elements in the two components also supports the idea that the enrichment must have occurred in each of the structures separately, and not as a continuum of events in a single GC. The most probable explanation is that the proto-clusters formed into a (now dissolved) dwarf galaxy and later merged to produce the present GC.