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 A _ { K _ { \mathrm { S } } } = 0.91 \pm 0.05 mag , an age \tau = 4 \pm 0.5 Myr and a distance d = 4.0 \pm 0.2 kpc for Westerlund 1 , as well as a total mass of M _ { \mathrm { Wd } 1 } = 4.91 _ { -0.49 } ^ { +1.79 } \times 10 ^ { 4 } M _ { \sun } .
Using spatially dependent completeness corrections we performed a 2D study of the cluster ’ s 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 10 ^ { 5 } 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 axis 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 .