  We present a general method to identify infalling substructure in discrete
datasets with position and line-of-sight velocity data. We exploit the fact
that galaxies falling onto a brightest cluster galaxy (BCG) in a virialised
cluster, or dwarf satellites falling onto a central galaxy like the Milky Way,
follow nearly radial orbits. If the orbits are exactly radial, we show how to
find the probability distribution for a satellite's energy, given a tracer
density for the satellite population, by solving an Abel integral equation.
This is an extension of Eddington (1916)'s classical formula for the isotropic
distribution function. When applied to a system of galaxies, clustering in
energy space can then be quantified using the Kullback-Leibler divergence, and
groups of objects can be identified which, though separated in the sky, may be
falling in on the same orbit. This method is tested using mock data and applied
to the satellite galaxy population around M87, the BCG in Virgo, and a number
of associations are found which may represent infalling galaxy groups.
