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 Msun (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 Delta_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 Msun in the cluster center (r <
10''). Applying a field star rejection and correcting for
incompleteness, we derive the Ks-band luminosity function (LF) for
stars simultaneously detected in the JHKs-bands. The LF follows a
power-law with an index of alpha ~ 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
Gamma ~ -0.74 in the mass range of $0.4 - 20 Msun. This is
substantially flatter than the Salpeter-like IMF (Gamma = -1.35).
The IMF power-law index decreases from Gamma ~ -0.31 at r < 5''
to Gamma ~ -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 x 10^4 Msun and 25'' - 50'' (~0.7
- 1.5 pc), respectively. The derived core radius is > 6 x 10^4
Msun pc^-3. The estimate of the half-mass relaxation time for stars
with a typical mass of 1 Msun is 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 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 intermediate- and low-mass stars in NGC 3603, and
the stars residing outside the observed field can merely steepen
the IMF by Delta_Gamma < 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 Gamma =
-0.74^{+0.62}_{-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.