Models for galaxy halos in an open universe

We present the properties of simple, self-consistent infall models which embed an " isothermal " dark halo in an open universe. These models depend on one scaling parameter and two shape parameters. The first may be taken to be the circular velocity in the inner regions, and the others to be the density of the universe, Ω, and a measure, e, of the shape of orbits. These models provide a useful description of ensembles of satellite galaxies, or of binary galaxies, because they allow explicitly for the fact that such systems have orbital periods approaching the age of the universe and so cannot have a random orbital phase.