Mass segregation and elongation of the starburst cluster Westerlund 1

Massive stellar clusters are the best available laboratories to study the mass function of stars. Based on NTT/SofI near-infrared photometry, we have investigated the properties of the massive young cluster Westerlund 1. From comparison with stellar models, we derived an extinction ? mag, an age τ= 4 ± 0.5 Myr and a distance d= 4.0 ± 0.2 kpc for Westerlund 1, as well as a total mass of ?. Using spatially-dependent completeness corrections, we performed a two-dimensional study of the cluster's initial mass function (IMF) and, in addition, of the stellar density profiles of the cluster as a function of mass. From both IMF slope variations and stellar density, we find strong evidence of mass segregation. For a cluster with some 105 stars, this is not expected at such a young age as the result of two-body relaxation alone. We also confirm previous findings on the elongation of Westerlund 1; assuming an elliptical density profile, we found an axial ratio of a:b= 3:2. Rapid mass segregation and elongation could be well explained as the results of subclusters merging during the formation of Westerlund 1. Based on observations collected at the European Southern Observatory, La Silla, Chile, and retrieved from the ESO archive (Program ID 67.C-0514)