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

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.