We present deep near-infrared HST/WFC3 observations of the young supermassive Galactic star cluster Westerlund 1 and an adjacent control field .
The depth of the data is sufficient to derive the mass function for the cluster as a function of radius down to 0.15 M _ { \odot } in the outer parts of the cluster .
We identify for the first time a flattening in the mass function ( in logarithmic units ) at a mass range that is consistent with that of the field and nearby embedded clusters .
Through log-normal functional fits to the mass functions we find the nominal peak mass to be comparable to that of the field and nearby embedded star clusters .
The width of a log-normal fit appears slightly narrow compared to the width of the field IMF , closer to the values found for globular clusters .
The subsolar content within the cluster does not appear to be mass segregated in contrast to the findings for the supersolar content .
The total mass of Westerlund 1 is estimated to be 44-57 \times 10 ^ { 3 } M _ { \odot } where the main uncertainty is the choice of the isochrone age and the higher mass slope .
Comparing the photometric mass with the dynamically determined mass , Westerlund 1 is sufficiently massive to remain bound and could potentially evolve into a low-mass globular cluster .