AAOmega Observations of 47 Tucanae: Evidence for a Past Merger?

TL;DR

This study uses Bayesian analysis on spectral data to provide strong evidence that 47 Tucanae is a result of a past merger of two populations, explaining its complex kinematic and stellar characteristics.

Contribution

It presents the first Bayesian evidence supporting a merger origin for 47 Tucanae based on spectral data analysis.

Findings

Bayesian analysis favors a two-component population model by a factor of ~3x10^7.

The merger likely occurred less than 7.3 Gyr ago.

The merger explains the cluster's rotation, stellar populations, and other properties.

Abstract

The globular cluster 47 Tucanae is well studied but it has many characteristics that are unexplained, including a significant rise in the velocity dispersion profile at large radii, indicating the exciting possibility of two distinct kinematic populations. In this Letter we employ a Bayesian approach to the analysis of the largest available spectral dataset of 47 Tucanae to determine whether this apparently two-component population is real. Assuming the two models were equally likely before taking the data into account, we find that the evidence favours the two-component population model by a factor of ~3x10^7. Several possible explanations for this result are explored, namely the evaporation of low-mass stars, a hierarchical merger, extant remnants of two initially segregated populations, and multiple star formation epochs. We find the most compelling explanation for the two-component velocity distribution is that 47 Tuc formed as two separate populations arising from the same proto-cluster cloud which merged <7.3 +/- 1.5 Gyr ago. This may also explain the extreme rotation, low mass-to-light ratio and mixed stellar populations of this cluster.