|Annu. Rev. Astron. Astrophys. 2000. 38: 289-335
Copyright © 2000 by Annual Reviews. All rights reserved
5.6. Gravitational Lensing
The efficiency of a massive system to act as a gravitational lens is a function of both the mass-density profile and the source-lens-observer geometry (cf Blandford & Narayan 1992). Given their relatively high mass densities, X-ray groups at moderate (z > 0.2) redshifts are expected to be efficient lenses (Mendes de Oliveira & Giraud 1994, Montoya et al 1996). Unfortunately, because very few samples of galaxy groups at moderate redshift exist in the literature, systematic searches for lensing in these objects have not been carried out. However, several of the well-studied, multiple-image QSO systems are lensed by galaxies that belong to spectroscopically-confirmed poor groups (Kundic et al 1997a, b; Tonry 1998; Tonry & Kochanek 2000). Although the primary lens in each of these cases is an individual galaxy, the group potential also contributes to the observed lensing. The presence of an extended group potential acts as a source of external sheer (Keeton et al 1997; Kundic et al 1997a, b). To properly model the lensing system, the group potential must be included. Most authors have attempted to measure the velocity dispersion of the group and then assume a form for the potential. Unfortunately, these dispersions are based on only a few velocity measurements and are subject to the large uncertainties that have plagued optical studies of nearby groups. Still, good fits to the lensing data are often obtained. In the case of the quadruple lens PG 1115+080, the measured velocity dispersion of the group (Kundic et al 1997a; Tonry 1998) is consistent with the value predicted earlier from the lensing data (Schecter et al 1997). Obtaining an adequate model for the group potential is also necessary to derive cosmological parameters like the Hubble Constant (Ho) from lensing experiments. Future X-ray observations may be the key to such techniques. High-resolution X-ray images taken with CHANDRA and XMM should allow the potential of the lensing groups to be mapped in detail. A better determination of the lensing potential will result in tighter constraints on cosmological parameters.