We have analysed high resolution adaptive optics ( AO ) science demonstration data of the young , massive stellar cluster Arches near the Galactic Center , obtained with the Gemini North telescope in combination with the University of Hawai ’ i AO system Hokupa ’ a .
The AO H and K ’ photometry is calibrated using HST/NICMOS observations in the equivalent filters F160W and F205W obtained by Figer et al .
( [ 1999 ] ) .
The calibration procedure allows a detailed comparison of the ground-based adaptive optics observations against diffraction limited space-based photometry .
The spatial resolution as well as the overall signal-to-noise ratio of the Gemini/Hokupa ’ a data is comparable to the HST/NICMOS data .
The low Strehl ratio of only a few percent is the dominant limiting factor in the Gemini AO science demonstration data as opposed to space-based observations .
After a thorough technical comparison , the Gemini and HST data are used in combination to study the spatial distribution of stellar masses in the Arches cluster .
Arches is one of the densest young clusters known in the Milky Way , with a central density of \sim 3 \cdot 10 ^ { 5 } M _ { \odot } { pc ^ { -3 } } and a total mass of about 10 ^ { 4 } M _ { \odot } .
A strong colour gradient is observed over the cluster field .
The visual extinction increases by \Delta A _ { V } \sim 10 mag over a distance of 15″ from the cluster core .
Extinction maps reveal a low-extinction cavity in the densest parts of Arches ( R \leq 5 \arcsec ) , indicating the depletion of dust due to stellar winds or photo-evaporation .
We correct for the change in extinction over the field and show that the slope of the mass function is strongly influenced by the effects of differential extinction .
We obtain present-day mass function slopes of \Gamma \sim - 0.8 \pm 0.2 in the mass range 6 < M < 65 M _ { \odot } from both data sets .
The spatial analysis reveals a steepening of the mass function slope from close to zero in the cluster center to \Gamma \sim - 1.7 \pm 0.7 at R > 10 \arcsec , in accordance with a Salpeter slope ( \Gamma = -1.35 ) .
The bias in the mass function towards high-mass stars in the Arches center is a strong indication for mass segregation .
The dynamical and relaxation timescales for Arches are estimated , and possible mass segregation effects are discussed with respect to cluster formation models .