The initial mass function and stellar content of NGC 3603

We present UBVRI and Halpha photometry of the extremely compact, starburst cluster NGC 3603. Ground-based images, Hubble Space Telescope (HST) archival data, as well as Chandra X-ray data have been used for this study. We present, for the first time, optical color-magnitude diagrams (CMDs) for the stars in the cluster core down to V = 22 mag. A well-defined main sequence ( MS) as well as low-mass pre-main-sequence (PMS) stars can clearly be seen in the CMDs. This result confirms the finding by Eisenhauer et al. that low-mass stars are forming in the starburst cluster. We also derive an age (1 +/- 1 Myr) and distance of the cluster (V-0 - M-V = 14.2 +/- 0.2 mag, i.e., d = 6.9 +/- 0.6 kpc). The interstellar reddening shows a minimum value [E( B - V) = 1.25 mag] at the core (r less than or equal to 0.'2) and increases rapidly up to E(B - V) approximate to 1.8 mag or even higher with increasing distance from the center, which suggests the presence of a wind-driven cavity near the cluster center as found by Frogel et al. and Clayton. We also determine the radius of the cluster (r approximate to 2') from the surface-density profile of bright stars and X-ray sources. To investigate the initial mass function ( IMF) of the cluster, we performed a completeness test for the core region. Using the theoretical mass-luminosity relation of MS as well as PMS stars, we derived the IMF of stars imaged with the HST/PC1. The surface density of stars in the core is enormously high and decreases gradually with increasing distance from the center. The slope of the IMF also shows radial variation: fairly flat in the core (Gamma = - 0.5 +/- 0.1 at r less than or equal to 0.'1) and gradually steepening (Gamma = -0.8 +/- 0.2 at r = 0.'1 similar to 0.'2 and Gamma = - 1.2 +/- 0.2 at r > 0.'2 of HST/ PC1). The stars in the halo region of NGC 3603 ( outside of HST/ PC1) seem to be slightly older ( age approximate to 5 Myr) and presumably formed before the stars in the core.