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the galaxy ngc 4550 , an e7/s0 lenticular galaxy in the virgo cluster , was discovered by rubin et al .
( 1992 ) to be a galaxy consisting of two coeval , coplanar , and counter - rotating stellar disks .
many other cases of counter - rotation are now known in ellipticals and also in spirals , but what makes the case of ngc 4550 particular ( kuijken , fisher , & merrifield 1996 ) is that the mass ratio of the counter - rotating disks is nearly @xmath0 . in the case of ellipticals counter - rotation
is so frequent ( e.g.  schweizer 1998 ) that the merger or accretion origins seem the most likely explanations for such kinematic misalignments .
the difficulty with a merger scenario for ngc 4550 is that strong disk mergers usually lead to ellipticals and the destruction of the disks , a `` truth '' often believed to be general since the seminal paper of toomre & toomre ( 1972 ) .
thakar & ryden ( 1996 ) have shown that over several gyr a series of well correlated small _ gaseous _ merger events can lead to a massive counter - rotating gaseous disk .
but then this scenario requires to preserve special correlations over several gyr , contrary to a single event .
thus , it remains open whether a single merger of two equal mass spirals can result in some circumstances to a galaxy like ngc 4550 .    in a forthcoming paper ( pfenniger & puerari 1998 ) we will present in more details the general conditions leading to moderate heating .
in addition to mass and energy , one must also consider the angular momentum in the final budget .
an input of specific angular momentum tends to cool the system , while inputs of specific energy and mass tend to heat it .
since ngc 4550 is a rare object , a generic process is not required ; exceptional conditions can be acceptable and should be expected .
after various considerations we have retained : * 1 . * a nearly circular orbit of the initial disks , supposing that the excess energy of galaxies coming from infinity has already been absorbed by some outer matter .
* initial disks with opposite spins ( see fig.1 ) .
counter - rotating disks are ideal for minimizing shocks in outer gaseous disks . also the retrograde disk is less affected by tidal interactions .
nearly or exactly coplanar disks in the orbital plane .
this may look a rather improbable situation , but a favorable factor to align the disk spins is to have initially flattened dark matter ( the torques on misaligned disks is then high ) .
several arguments , such as the high frequency of warped outer hi disks with straight line of nodes , do suggest flattened dark mass distributions in spirals ( see pfenniger & combes 1994 ) .
the simulations presented here were run in geneva .
independently , puerari run similar simulations in mexico .
we intend to publish jointly both sets of simulations since they lead to the same conclusions despite different choices of initial parameters . in particular puerari s simulations include round massive halos , showing that the shape of the halos does not change the qualitative results , and his disks start on more elongated orbits .    in view of the poor understanding of gaseous processes in galaxies leading to exaggerate viscosity with the sph technique ( see thakar & ryden 1996 ) , we simulate first only the collisionless gravity part .
obviously the resulting heating is enhanced with respect to simulations including gas .
our two initial disks are identical , except for the initial coordinates .
they consist of a bulge , with a scale - length of 1kpc , an exponential disk with horizontal and vertical scale - lengths of 3 and 0.5kpc , and a flaring ( @xmath1 ) massive `` dark '' disk with a constant surface density up to 10kpc , decreasing as @xmath2 between 10 and 30kpc , and as @xmath3 between 30 and 60kpc
. the mass ratios of the three components are respectively ( 0.25:1:5 ) , and the particle masses are all equal . the particle velocities are found by solving the jeans equation for each component in the total potential , assuming velocity dispersions ellipsoids parallel to cylindrical coordinates .
the potential of the mass distribution is calculated with a set of @xmath4 particles on the geneva pm polar grid code .    a rapid run of coplanar counter - rotating disks merger is performed with the pm code with @xmath5 particles .
the result is positive but not fully convincing since during the merging process substantial changes of positions of the mass distribution occur within the grid , which has a position dependent resolution .
therefore , we select out a subset of @xmath6 particles , the mass of which is multiplied by a factor 20 to keep the same total mass
. the particle subset is then run with the barnes - hut ( 1989 ) treecode ( with an opening angle of @xmath7=0.5 ) , which does not imply any geometrical assumptions . in several experiments we verify that slight initial disk inclinations with the orbital plane ( 5@xmath8 )
still lead to counter - rotating coplanar disks .
here we just describe the strictly coplanar disk merger .
1 , left , shows the initial particle distribution with the senses of rotation .
the disks are on a prograde near circular orbit .
the tidal perturbation creates immediately ( at @xmath9200myr ) a bar in each disks , persisting until the disks merge .
the merging process conserves the disks fairly well , the less damaged one being the retrograde one .
fig .  1 ,
middle , shows the inner remnant disk at @xmath10=1500myr , still containing 80% of the initial total mass , almost circular and barless .
fig .  1 , right , shows the large - scale particle distribution , at @xmath10=1500myr .
the excess angular momentum is transported by 10 - 20% of the mass , mostly of the prograde disk , to large distances , 100500kpc .
fig .  2 shows how the average line profiles as seen in the edge - on disks change from unimodal to bimodal .
the bimodal distribution is crucial in order to distinguish counter - rotating disks from a hot population with a zero net rotation
. the ratios of final to initial velocity dispersion amount to @xmath92.4 in the radial direction , and @xmath92 in the vertical direction .
the final mass distribution follows well a @xmath11 profile . with a relatively limited heating the final disk resembles both in shape and kinematics a typical s0/e7 galaxy , with
, in addition , marked counter - rotating populations .    at some moments during the merging process the system as seen edge - on looks like a single galaxy but with two bulges
such a case of `` two - bulge '' looking galaxy is pgc 57064 in the hercules cluster .
we have shown that counter - rotating co - spatial disks can be made by a single spiral - spiral merger .
the required initial conditions are somewhat peculiar , but favored if the dark matter distribution is flat ; however , this is not a necessary condition .
the strong reaction of the disks , expelling 1020% of the mass to 100500kpc , means that dark matter in such systems must be distributed in a pronounced asymmetric way for several gyr .
thus , in many spirals the outer ( dark ) mass distribution should still be chaotic .
a single merger scenario for making galaxies such as ngc 4550 possessing equal coplanar counter - rotating stellar disks is investigated by collisionless n - body technique . 
 the scenario is successful in producing an axisymmetric disk made of two almost equal counter - rotating populations . 
 the final disk shows a clear bimodal line profile in the outer part , which demonstrates that disk - disk mergers do not always produce ellipticals .    
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