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An alternative approach to rejecting
spurious halos is to use the actual particle velocities
to directly calculate whether putative halos are gravitationally bound.
We do this using an ``evaporative'' method which is almost the
same as that used in the DENMAX algorithm [3, 6].
In this method each putative halo is considered in isolation,
and the total energy of each particle is calculated:
its gravitational potential energy in the field of the other particles,
plus its kinetic energy relative to the center of mass of
the halo. Then the particle with the highest energy is considered.
If the particle's total energy is negative then so are all particles
with lower energy, so the ensemble is guaranteed to be gravitationally
bound, and the halo is accepted.
Otherwise the particle's energy is positive -- meaning that it is on
an orbit that will eventually remove it from the neighborhood of the halo.
The particle is removed from the halo (``evaporated'') and
the process is repeated, recalculating the energies of the remaining
particles to account for the loss of the removed particle.
The process continues
until either the halo is accepted, or the number of particles
remaining drops below some minimum, in which case the halo is rejected.
(In the results included in this paper the minimum number of
particles is set to 10.)
The evaporative method can be considered more correct that
the simple and methods (in terms of directly
addressing our definition of a halo), but it does involve
significantly more computation. For each halo containing bodies,
evaporation requires operations.
To save time, we assume that halos with very many particles
(say ) are bound, since this is almost certainly the case.
Results using the evaporative method in Model 3 are shown
in the right hand panels of Figure 3.
The method is very good at discriminating between genuine and
spurious halos, and finds eleven genuine halos compared with only eight
found by .

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*John Salmon *

Sat Sep 27 18:44:36 PDT 1997