The IMF of the massive star-forming region NGC 3603 from NIR adaptive optics observations

We study the initial mass function (IMF) of NGC 3603, one of the most massive galactic star-forming regions, to answer a fundamental question in current astrophysics is the IMF universal, or does it vary? Using our very deep high angular resolution images obtained with the NAOS-CONICA adaptive optics system at the VLT/ESO, we have successfully revealed the low-mass stellar population in the cluster core down to about 0.4 M (50 % completeness limit). Based on the JHKSL ′ color-magnitude and color-color diagrams, we first derive an average age 0.7 Myr for the pre-main sequence stars, and an upper limit of ∼ 2.5 Myr for the main sequence stars. We find an average foreground extinction of AV = 4.5 ± 0.5 mag, with a radial increase of ∆AV ∼ 2.0 mag towards larger radii (r . 50). From the infrared excess emission identified in the KS − L vs J −H color-color diagram, we measure a disk fraction of ∼ 25 % for stars with M ≥ 0.9 M in the cluster center (r ≤ 10). Applying a field star rejection and correcting for incompleteness, we derive the KSband luminosity function (LF) for stars simultaneously detected in the JHKS-bands. The LF follows a power-law with an index of α ∼ 0.27, and shows no turnover or truncation within the detection limit. The IMF for stars within r ≤ 110 is reasonably fitted by a single power-law with index Γ ∼ −0.74 in the mass range of 0.4 − 20 M . This is substantially flatter than the Salpeter-like IMF (Γ = −1.35). The IMF powerlaw index decreases from Γ ∼ −0.31 at r ≤ 5 to Γ ∼ −0.86 at 30 < r ≤ 110. This radial steepening of the IMF mainly occurs in the inner r . 30 field, indicating mass segregation at the very center of the starburst cluster. Analyzing the radial mass density profile, we derive a cluster core radius of ∼ 4 .8 (∼ 0.14 pc), and a lower limit of ∼ 110 (∼ 3.2 pc) for the cluster size. We also derive an upper limit of r ∼ 1260 (∼ 37 pc) for the cluster size adopting an estimate of the tidal radius of the cluster. Based on the de-projected stellar density distribution, we estimate the total mass and the half-mass radius of NGC 3603 to be about 1.0 − 1.6 × 104 M and 25 − 50 (∼ 0.7 − 1.5 pc), respectively. The derived core radius is ≥ 6 × 104M pc−3. The estimate of the half-mass relaxation time for stars with a typical mass of 1 M is 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 is expected to be much smaller, and is comparable to the age of the cluster. We can thus not conclude if the mass segregation of the high-mass stars is primordial or caused by dynamical evolution. Our observation covers at least ∼ 67 % of intermediateand low-mass stars in NGC 3603, and the stars residing outside the observed field can merely steepen the IMF by ∆Γ ≤ 0.16. Therefore, because of the almost constant IMF beyond a radius r & 30, we are confident that our IMF adequately describes the whole NGC 3603 starburst cluster. We also thoroughly analyze the systematic uncertainties in our IMF determination. We conclude that the power-law index of NGC 3603 including the systematic uncertainties is Γ = −0.74 −0.47. Our result thus supports the hypothesis of a top-heavy IMF in starbursts, especially in combination with other studies of similar clusters such as the Arches cluster and the Galactic Center cluster.