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in the merger remnant system ngc  7252 maraston et al .
( 2004 ) found the most luminous star cluster known to date , w3 .
they determined its mass via two independent ways .
first they used the total luminosity of @xmath8 and a stellar @xmath9 derived from a single stellar population model to estimate its mass to be @xmath10  m@xmath2 . as an independent approach they calculated the dynamical mass by measuring the velocity dispersion , which turned out to be quite high , @xmath11  kms@xmath5 , and the effective radius of w3 @xmath12  pc .
this translates to a mass of w3 of @xmath13  m@xmath2 assuming dynamical equilibrium of a spherical object with isotropic velocity distribution .
the age of this object is about @xmath14@xmath15  myr ( maraston et al .
2004 and references therein ) , which indicates that it probably formed during the merger event of the host system .
the projected distance of w3 to the centre of ngc  7252 is about @xmath16  kpc and w3 appears to lie within the optical radius of the galaxy .
the size and mass of this object leads to the suggestion that it may be one of the recently discovered ultra compact dwarf galaxies ( ucd ) found in the fornax cluster ( hilker et al .
1999 ; phillipps et al .
2000 ) , rather than an ordinary globular cluster .
the ucd galaxies have been suggested to be the cores of stripped nucleated dwarf galaxies ( bekki et al .
2003 , mieske et al .
2004 ) .
we propose instead a formation scenario which is closely related to the massive star - bursts caused by the interaction of two gas - rich disc galaxies . in interacting systems like the antennae ( ngc  4038/39 ; whitmore et al .  1999 ,
zhang & fall 1999 ) regions of very intense star - formation arise as a result of the tightly compressed interstellar media .
dozens and up to hundreds of young massive star clusters are observed to form in star cluster complexes ( or _ super - clusters _ ) spanning up to a few hundred pc in diameter .
kroupa ( 1998 ) argues that these super - clusters have to be bound objects because their age ( @xmath17  myr ) indicates that they should be already dispersed . simulating super - clusters , by means of stellar dynamical @xmath18-body simulations , fellhauer et al .
( 2002 ) found that the star clusters within these super - clusters merge on very short time - scales ( a few dozens to a few hundred myr ) , namely a few crossing - times of the super - cluster .
the age - estimate of w3 points to the possibility that it may have formed during the merger event of the host - galaxy ( i.e.  out of merging star clusters ) rather than it being the stripped core of a dwarf galaxy , which should be very old .
in this project we perform stellar dynamical n - body simulations to show that the merging of star clusters in dense star cluster complexes is able to form massive objects like w3 in ngc  7252 .
the simulations are carried out with the particle - mesh code superbox with high - resolution sub - grids which stay focussed on the simulated objects ( fellhauer et al .
2000 ) .    in our models the super - cluster
is initially represented by a plummer sphere with a plummer radius of @xmath19  pc and a cut - off radius of @xmath15  pc .
the clusters inside this super - cluster have a total mass of @xmath20  m@xmath2 , which leads to a crossing time of the super - cluster of @xmath21  myr . inside the plummer sphere of the super - cluster ,
the particles have positions and velocities according to the plummer distribution function .
the particles themselves are plummer spheres representing the star clusters with , respectively , plummer radii of @xmath22 or @xmath16  pc , masses of @xmath23 and @xmath24  m@xmath2 and crossing times of @xmath25 and @xmath26  myr , being represented by @xmath27 and @xmath28  particles . altogether the super - cluster is filled with @xmath29 star clusters , from which @xmath30 are light ones and @xmath31 are of the heavy type to mimic a mass spectrum similar to the one found in the young massive star clusters in the antennae ( zhang & fall 1999 ) .    this super - cluster
is now placed into an analytical galactic potential consisting of a logarithmic potential for the underlying halo , a plummer - kuzmin disc and a hernquist bulge , which add up to an almost flat rotation curve of @xmath32  kms@xmath5 .
the super - cluster is placed at a distance of @xmath33  kpc initially ( apogalacticon ) on an eccentric orbit with perigalacticon of @xmath16  kpc .
= 03.2 cm = 03.2 cm = 03.2 cm = 03.2 cm = 03.2 cm = 03.2 cm = 03.2 cm = 03.2 cm = 03.2 cm = 03.2 cm = 03.2 cm = 03.2 cm = 03.2 cm = 03.2 cm = 03.2 cm = 03.2 cm = 03.2 cm = 03.2 cm    in contrast to our previous models ( fellhauer & kroupa 2002a , b ) where the merger objects were smaller and not that heavy , the merging process here takes more time . after the first clusters have merged and build up a massive and extended object with the remaining clusters within , it is not encounters between clusters and the merger object which govern the merging process ( the clusters are already within the object ) but dynamical friction acting on the remaining clusters .
nevertheless a massive merger object is present from an early stage on .
the top row of figure  [ fig : t300 ] shows the contour plot of the merger object ( left at @xmath34 and right at @xmath35  myr comprising the suggested age - range of w3 ) .
the contours are spaced in magnitude intervals and masses are converted to luminosities taking a mass - to - light ratio of 0.15 . according to a single stellar population model run with starburst99 ( leitherer et al .
1999 ) a stellar population of this age has a @xmath36@xmath37 . in our model these two time - slices mark the transition between the time , when still many star clusters are visible as separate entities and the time when most of the star clusters have already merged or are dissolved within the merger object .
another interesting feature of our model is that the merger object shows a cuspy structure with a dynamically cold core .
the surface - density profile can be fitted by either two king profiles or two exponentials ( fig .  [
fig : t300 ] , middle row ) and the velocity dispersion is rising in the innermost part and reaches its maximum beyond the transition between the core and the envelope ( fig .  [
fig : t300 ] , bottom row ) .
the outer part of the velocity dispersion , reaching from the maximum value to the tidal radius , can be fitted with an exponential profile with an exponential scale length of the order of the tidal radius of the object . beyond the tidal radius the velocity dispersion is rising again due to the large velocities of the extra tidal stars .    in fig .
[ fig : evol ] we show the evolution of the total mass , of the effective radius and of the velocity dispersion of our model , whose dynamical evolution was followed for @xmath38  gyr . shown in the right panel of the figure
are the central values of the dynamically cold core as well as the maximum values of the velocity dispersion .
the boxes show the values measured for w3 .
the total mass as well as the maximum velocity dispersion of our model is in reasonable agreement with the data of w3 .
only the effective radius of our model is too small compared with w3 .
it starts off at a value of about @xmath16  pc and it increases slightly until the majority of the star clusters have merged , deforming the core progressively .
after the merging process has ceased the core of the merger object stabilises and the effective radius drops to about @xmath38  pc . but note the extremely large value at @xmath39  myr . at exactly that time a late merger event of one of the remaining star clusters happened .
if our formation scenario is correct then this points to the possibility that we might be observing w3 at the time when a star cluster merges with the core of the object mimicking a large effective radius .
= 03.2 cm = 03.2 cm = 03.2 cm = 03.2 cm = 03.2 cm = 03.2 cm    the further dynamical evolution is mainly governed by the tidal shaping of the object due to its eccentric orbit .
it leads to a successive mass loss and an increase in the effective radius as well as a decrease in the maximum velocity dispersion , while the central value increases slightly .
looking at the merger object after @xmath38  gyr of its dynamical evolution ( fig .
[ fig : t300 ] ) shows that this object is very stable against tidal disruption .
also the dynamically cold core is not a transient feature but survives the evolution .
the total mass of the object is still of the order of @xmath40  m@xmath2 which is an order of magnitude more than the most massive globular cluster ( @xmath41  cen ) of the milky way .
the relaxation time - scale of this object , adopting the formula from spitzer & hart ( 1971 ) , is @xmath42  gyr .
this shows that this object might to be thought of as a dwarf galaxy rather than a globular cluster , since @xmath43 is much longer than a hubble time .
with our numerical model we introduce a formation scenario for the ultra - massive star cluster w3 found in ngc  7252 . we propose that this object may not have formed as one star cluster , but rather as a star cluster complex .
the star clusters in this complex have merged and formed w3 .
these star cluster complexes ( super - clusters ) are a common feature in interacting late - type galaxies .
we predict how w3 should look like if it would be possible to resolve it .
its age places it at about the transition time between having many remaining star clusters visible inside and the time when all star clusters have already merged .
one should see the last surviving star clusters in the stage of merging .
furthermore a merger object as massive as w3 should show a distinct core 
envelope structure .
the merger object in our calculation had a dense , dynamically cold core and an extended envelope .
this core 
envelope structure is not a transient feature but a stable configuration .
we followed the evolution of our merger object for @xmath38  gyr and this structure remained .
the core is formed out of the merged star clusters .
the envelope consists out of the stripped stars , lost by the star clusters during the initial violent merger process . in our previous models the merger objects are not massive enough to keep stripped stars bound and they are lost . in the case of the w3 model
the core object is massive enough to retain the stripped stars bound , forming the envelope structure of our model .
w3 is said to be one of the newly discovered ultra - compact dwarf ( ucd ) galaxies and also one of the most massive ones ( maraston et al.2004 ) .
other ucds are found and studied around the central galaxy in the fornax cluster ( ngc  1399 ) by hilker et al .
( 1999 ) , phillipps et al .
( 2001 ) and mieske et al .
there these objects are quite old .
there are two competing formation scenarios for ucds . while bekki et al .
( 2003 ) propose that these objects are the remaining cores of stripped nucleated dwarf galaxies , fellhauer & kroupa ( 2002a ) proposed the merging scenario from star cluster complexes as a possible formation process , resulting naturally from the merging of gas - rich galaxies in groups as the groups merge with a galaxy cluster .
a possible way to distinguish between the two scenarios would be an analysis of the stellar populations in these objects .
while the merging star cluster scenario implies that the stars of the ucds have more or less the same metallicity and age ( this holds at least for the most prominent population formed in the starburst ) , cores of dwarf galaxies should show a more complex metallicity and age distribution . and
as dwarf galaxies are believed to be the oldest building blocks in the universe , the cores should show a prominent very old population .    in the case of
w3 the stripping scenario can be completely ruled out .
age estimates of this object range from @xmath14 to @xmath15  myr , which is also the age of the interaction ( ngc  7252 is a merger remnant of two gas - rich disc galaxies ) .
this time is much too short for a nucleus to be stripped of its surrounding dwarf galaxy .
if this implies that all ucds must have formed in the same way or if both scenarios are possible and are realized by nature can not yet be asserted .
the young star cluster w3 ( age @xmath0  myr ) in ngc  7252 is the most luminous star cluster known to date . 
 dynamical mass estimates result in @xmath1  m@xmath2 . with an effective radius of about @xmath3 parsec and a velocity dispersion of @xmath4  kms@xmath5 this object is rather one of the recently discovered ultra - compact dwarf ( ucd ) galaxies than a star cluster .    in our model 
 we propose a formation scenario for w3 . 
 observations of interacting galaxies reveal regions of strong star formation forming dozens up to hundreds of star clusters in confined regions of several hundred parsec in diameter . 
 the total mass of new stars in these regions can reach @xmath6 or even @xmath7 solar masses . by means of numerical simulations 
 we have shown that the star clusters in these regions merge on short time - scales forming objects like ucd galaxies or w3 .