  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). ... 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. ... A strong colour gradient is observed over the
cluster field. The visual extinction increases by Delta A_V ~ 10 mag over a
distance of 15 arcsec from the cluster core. Extinction maps reveal a
low-extinction cavity in the densest parts of Arches (R < 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=-0.8 +- 0.2 in the mass range
6 < M < 65 Msun from both data sets. The spatial analysis reveals a steepening
of the mass function slope from close to zero in the cluster center to about
-1.7 +- 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.
