Gravitational lensing and dark structures

We examine whether a cosmologically significant distribution of dark galaxy group or cluster-sized objects can have an optical depth for multiple imag ing of distant background sources, which is comparable with that from known galaxies while at the same time producing angular splittings of the same o rder of magnitude. Our purpose is to explore whether such objects could rea listically account for some of the observed lenses. Modeling such systems a s isothermal spheres with core radii and assuming a Schechter-type distribu tion function, we find that independent of the cosmology (open, flat matter -dominated, or flat cosmological constant-dominated) an allowed, albeit nar row, parameter range exists that is comparable in velocity dispersion with that for known compact groups of galaxies, although the preferred core radi i are somewhat smaller than that normally assumed for compact groups. Dark cluster-sized objects, on the other hand, cannot reproduce the observed len sing characteristics. If the one known dark cluster were a good representat ive of such a distribution, most such objects would not produce multiple im ages. We also present a result for the angular splitting from an isothermal sphere lens with nonzero core radius, extending earlier work of Hinshaw & Krauss. Our results are expressed as contour plots for fixed lensing probab ilities and angular splittings.