AAOMEGA OBSERVATIONS OF 47 TUCANAE: EVIDENCE FOR A PAST MERGER?

The globular cluster 47 Tucanae (47 Tuc) 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 data set of 47 Tuc 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 favors the two-component population model by a factor of ∼3 x 107. 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.