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Paper text:
the arches cluster is one of the most massive young star clusters found in the milky way .
the total stellar mass is estimated to be @xmath1 , with a central stellar density of @xmath2 ( figer et al .
located at a projected distance of only 25 pc from the galactic center ( gc ) , it evolves under extreme physical conditions . as the formation of massive stars ( @xmath3 )
is proposed to be strongly dependent on high stellar and gas densities ( behrend & maeder 2001 , bonnell et al .
1998 ) , star clusters close to the gc are important test beds for star formation ( sf ) scenarios in dense regions . displaying the highest stellar density found in a massive young cluster ( yc ) in the milky way , the arches cluster is a unique object for the comparison with massive sf models . due to the proximity to the gc , strong tidal forces act towards the disruption of the cluster entity , causing the timescale for dynamical evolution to be short , which may lead to the dissolution of the cluster within @xmath4 ( kim et al .
1999 , portegies zwart et al .
2002 ) . with an age of only @xmath5 myr ( figer et al .
2002 , in prep . )
, the stellar population of the arches cluster should be mostly unaffected by stellar evolution , but the spatial appearance of the cluster should reflect the fast dynamical evolution .
we analyse high - resolution adaptive optics ( ao ) @xmath6 and @xmath7-band observations of the arches cluster center obtained during the gemini north science verification ( sv ) with the university of hawaii ao system hokupaa ( graves et al . 2000 ) .
these data are compared to hst / nicmos observations of the same field obtained by figer et al .
( 1999 ) . in uncrowded regions , the spatial resolution and limiting magnitudes in both datasets are comparable given a 3 to 4 times longer integration time in @xmath6 and @xmath7 , respectively , for the gemini observations .
in crowded regions , the ao data are limited by uncompensated seeing haloes from bright stars caused by the low strehl ratio of only a few percent in the gemini sv data .
these haloes locally decrease the detection efficiency of faint sources .
evidence for this is seen at the faint end of the colour - magnitude diagrams and in the mf of the dense cluster center .
the @xmath8 colour - magnitude diagrams ( cmds ) are shown in fig .
1 . the dominant feature in the cmds is the arches main sequence ( ms ) . due to the low strehl ratio in the gemini data
, the upper part of the ms , corresponding to the bright population in the cluster center , appears less confined than in the nicmos cmd .
the fainter stars , not detected in the cluster center due to crowding , show a large scatter towards redder colours , which is caused by a strong colour gradient observed over the arches field .
the colour trend is apparent in both datasets , with increasingly red stellar colours observed at increasing radii ( fig .
2 ) . the change in @xmath9 corresponds to an increase in visual extinction of @xmath10 mag ( @xmath11 , rieke & lebofsky ( 1985 ) ) .
the well confined main sequence at the bright end of the cmd shows that the innermost cluster center ( @xmath12 , 0.2 pc at 8 kpc distance ) is unaffected by this colour trend .
this suggests that the remaining dust and gas has been expelled from the cluster center by winds and/or photoevaporation from the massive stars .
after correction for the extinction variation , a 2 myr isochrone from the geneva basic set of stellar evolution models ( lejeune & schaerer 2001 ) has been used to transform @xmath13-band magnitudes into stellar masses .
the resulting mf ( fig .
3 ) displays a slope of @xmath0 in the mass range @xmath14 in the gemini as well as the hst dataset .
this slope is flatter than the salpeter slope of @xmath15 , often assumed to be universal in young star forming regions .
when neglecting the colour trend , we derive a mf slope as flat as @xmath16 .
this clearly shows that the changes in extinction in sf regions are not negligible when deriving mass functions .
the flat mf observed in arches indicates an overdensity of high - mass stars as compared to other sf regions , thereby supporting models predicting enhanced high - mass sf in dense regions .
we have analysed the stellar population at different radii from the cluster center to derive the radial variation in the mf ( fig .
4 ) . the mf slope appears very flat in the cluster center , where stellar and gas densities are highest , and increases with increasing radius . while mainly massive stars are found in the central part of arches , an increasing number of faint , low - mass stars is found in the cluster outskirts , indicating mass segregation . to exclude that the derived variation is due to crowding
, incompleteness simulations have been performed in each radial bin , shown as dotted lines in figs . 3 and 4 .
although incompleteness begins to affect the mf earlier in the cluster center ( @xmath17 ) , the corrected mf remains very flat for @xmath18 . from the distribution of stellar masses
, we can obtain a rough estimate of the relaxation timescale , @xmath19 , where @xmath20 is the mass within the half - mass radius @xmath21 , @xmath22 the characteristic mass , and n the number of stars ( binney & tremain 1987 ) .
we obtain @xmath23 and observe @xmath24 as a lower limit to the total mass .
the median mass within @xmath21 is measured to be @xmath25 . assuming a total mass of at least @xmath26 leads to @xmath27 . a characteristic mass of @xmath28 , more typical for sf regions , yields @xmath29 .
with @xmath30 being on the same order of magnitude as the age of arches , it is impossible to discrimate between primordial and dynamical mass segregation .
the radial variation in the mf of the arches cluster as well as the flat integrated mass function support massive cluster and star formation models , suggesting high - mass stars to form in dense cluster environments .
although the radial variation in the mf strongly suggests mass segregation in arches to be present , the short dynamical timescales , strongly influenced by the gc tidal field , prohibit discrimination between primordial and dynamical segregation effects .
the close similarity of the mass functions obtained from the gemini ao and the hst datasets reveal that high - resolution ground - based ao data are capable to produce comparable physical results as space - based observations - in particular in dense regions where spatial resolution is essential .
we present the mass function ( mf ) of the arches cluster obtained from ground - based adaptive optics data in comparison with results derived from hst / nicmos data . 
 a mf slope of @xmath0 is obtained . 
 both datasets reveal a strong radial variation in the mf , with a flat slope in the cluster center , which increases with increasing radius .