The initial mass function of the massive star-forming region NGC 3603 from near-infrared adaptive optics observations

We study the initial mass function ( IMF) of one of the most massive Galactic star-forming regions NGC 3603 to answer a fundamental question in current astrophysics: is the IMF universal, or does it vary? Using our very deep, high angular resolution JHK(S)L(1) images obtained with NAOS-CONICA at the VLT at ESO, we have successfully revealed the stellar population down to the subsolar mass range in the core of the starburst cluster. The derived IMF of NGC 3603 is reasonably fitted by a single power law with index Gamma similar to -0:74 within a mass range of 0.4-20 M-circle dot, substantially flatter than the Salpeter-like IMF. A strong radial steepening of the IMF is observed mainly in the inner r less than or similar to 30 '' field, indicating mass segregation in the cluster center. We estimate the total mass of NGC 3603 to be about 1: 0-1: 6 x 10(4) M-circle dot. The derived core density is >= 6 x 10(4) M(circle dot)pc(-3), an order of magnitude larger than, e. g., the Orion Nebula Cluster. The estimate of the half-mass relaxation time for solar-mass stars is about 10-40 Myr, suggesting that the intermediate- and low-mass stars have not yet been affected significantly by the dynamical relaxation in the cluster. The relaxation time for the high-mass stars can be comparable to the age of the cluster. We estimate that the stars residing outside the observed field cannot steepen the IMF significantly, indicating our IMF adequately describes the whole cluster. Analyzing thoroughly the systematic uncertainties in our IMF determination, we conclude that the power-law index of the IMF of NGC 3603 is Gamma similar to -0.74(-0.47)(+0.62). Our result thus supports the hypothesis of a potential top-heavy IMF in massive star-forming clusters and starbursts.