( catalog Westerlund 1 ) is the most important starburst cluster in the Galaxy due to its massive star content .
We have performed BVI _ { C } and JK _ { S } photometry to investigate the initial mass function ( IMF ) .
By comparing the observed color with the spectral type - intrinsic color relation , we obtain the mean interstellar reddening of < E ( B - V ) > = 4.19 \pm 0.23 and < E ( J - K _ { S } ) > = 1.70 \pm 0.21 .
Due to the heavy extinction toward the cluster , the zero-age main sequence fitting method based on optical photometry proved to be inappropriate for the distance determination , while the near-infrared photometry gave a reliable distance to the cluster – 3.8 kpc from the empirical relation .
Using the recent theoretical stellar evolution models with rotation , the age of the cluster is estimated to be 5.0 \pm 1.0 Myr .
We derived the IMF in the massive part and obtained a fairly shallow slope of \Gamma = -0.8 \pm 0.1 .
The integration of the IMF gave a total mass for the cluster in excess of 5.0 \times 10 ^ { 4 } M _ { \sun } .
The IMF shows a clear radial variation indicating the presence of mass segregation .
We also discuss the possible star formation history of Westerlund 1 from the presence of red supergiants and relatively low-luminosity yellow hypergiants .