  We study equilibria in domains with boundaries for a first-order aggregation
model that includes social interactions and exogenous forces. Such equilibrium
solutions can be connected or disconnected, the latter consisting in a delta
concentration on the boundary and a free swarm component in the interior of the
domain. Equilibria are stationary points of an energy functional, and stable
configurations are local minimizers of this functional. We find a one-parameter
family of disconnected equilibrium configurations which are not energy
minimizers; the only stable equilibria are the connected states. Nevertheless,
we demonstrate that in certain cases the dynamical evolution, along the
gradient flow of the energy functional, tends to overwhelmingly favour the
formation of (unstable) disconnected equilibria.
