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 ^ { \prime } 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 \sim - 0.74 within a mass range of 0.4 - 20 M _ { \odot } , substantially flatter than the Salpeter-like IMF .
A strong radial steepening of the IMF is observed mainly in the inner r \lesssim 30 ^ { \prime \prime } field , indicating mass segregation in the cluster center .
We estimate the total mass of NGC 3603 to be about 1.0 - 1.6 \times 10 ^ { 4 } M _ { \odot } .
The derived core density is \geq 6 \times 10 ^ { 4 } M _ { \odot } 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 can not 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 = -0.74 ^ { +0.62 } _ { -0.47 } .
Our result thus supports the hypothesis of a potential top-heavy IMF in massive star-forming clusters and starbursts .