  We present a new definition of subhalos in dissipationless dark matter N-body
simulations, based on the coherent identification of their dynamically bound
constituents. Whereas previous methods of determining the energetically bound
components of a subhalo ignored the contribution of all the remaining particles
in the halo (those not geometrically or dynamically associated with the
subhalo), our method allows for all the forces, both internal and external,
exerted on the subhalo. We demonstrate, using the output of a simulation at
different timesteps, that our new method is more accurate at identifying the
bound mass of a subhalo. We then compare our new method to previously adopted
means of identifying subhalos by applying each to a sample of 1838 virialized
halos extracted from a high resolution cosmological simulation. We find that
the subhalo distributions are similar in each case, and that the increase in
the binding energy of a subhalo from including all the particles located within
it is almost entirely balanced by the losses due to the external forces; the
net increase in the mass fraction of subhalos is roughly 10%, and the extra
substructures tending to reside in the inner parts of the system. Finally, we
compare the subhalo populations of halos to the sub-subhalo populations of
subhalos, finding the two distributions to be similar. This is a new and
interesting result, suggesting a self-similarity in the hierarchy substructures
within cluster mass halos.
