Infrared observations of NGC 3603*

We have performed deep Ks-band observations of the starburst region NGC 3603 with the infrared camera ISAAC mounted on the VLT Antu. The total area covered by our data stretches from the NGC 3603 starburst cluster towards the south up to a maximal distance of about 370$\hbox{$^{\prime\prime}$}$(~12 pc). This enables us to reconsider and redetermine the radial extent of the cluster which is found to be about 150$\hbox{$^{\prime\prime}$}$±15$\hbox{$^{\prime\prime}$}$(~5 pc), exceeding previous estimates by a factor of 2.5. King model fits are used to disentangle the count statistics of cluster stars from those of field stars. With knowledge of the cluster radius we then construct and analyze the Ks-band luminosity function (KLF) of the NGC 3603 starburst cluster. The KLF for cluster radii >30$\hbox{$^{\prime\prime}$}$is rising down to the completeness limit of our study at Ks~ 17$\hbox{m}$–17$\hbox{m}\!\!\!.\,$5, corresponding to 1 Myr old cluster members of $\cal M$~ 0.5 $\cal M\hbox{$_{\odot}$}$. For the range of intermediate luminosity stars we obtain a KLF slope of α= 0.35 ±0.02 which is consistent with a Miller-Scalo type initial mass function (IMF) of a ~1 Myr old stellar cluster if one assumes a power-law index of β~ 2 for the mass-luminosity relation of both intermediate mass main sequence stars and low mass pre-main sequence stars. At the high luminosity end the observed KLF likely flattens (α~ 0.2), being consistent with a Salpeter type IMF for NGC 3603's massive main sequence stars.