The Imf of the Massive Star-forming Region Ngc 3603 from Nir Ao 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 JHKSL ′ images obtained with the AO-assisted NAOS-CONICA instrument at the VLT/ESO, we have successfully revealed the low-mass 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 Γ ∼ −0.74 within a mass range of 0.4 − 20 M⊙, substantially flatter than the Salpeter-like IMF. A strong radial steepening of the IMF is observed in the inner r . 30 field, indicating mass segregation at the very center of the cluster. We estimate the total mass of NGC 3603 to be about 1.0 − 1.6 × 10 M⊙. The derived core density is ≥ 6× 10M⊙ pc, 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 intermediateand 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. Thus the mass segregation observed through the radial variation of the IMF is likely due to high-mass stars. We estimate that the stars residing outside the observed field can not steepen the IMF significantly, indicating our IMF adequately describes the whole NGC 3603 starburst cluster. Analyzing thoroughly the systematic uncertainties in our IMF determination, we conclude that the power-law index of the IMF of NGC 3603 including the systematic uncertainties is Γ = −0.74 −0.47. Our result thus supports the hypothesis of a potential top-heavy IMF in massive star-forming clusters and starbursts. Subject headings: stars: luminosity function, mass function — stars: pre-main sequence — stars: formation — HII regions — open clusters and associations: individual (NGC 3603)