3.2 Strong Gravitational Lenses

A number of strong (elliptical galaxy) gravitational lens systems are known that may offer the potential of constraining the value of _m and through modeling of the lens properties. This method is less sensitive to than the statistics of lensing, and again it is sensitive to a number of possible systematic effects: possible perturbations by cluster potentials, uncertainties in the underlying properties of the lensing galaxies, and model-dependent corrections due to evolution. The objects are faint and the errors in the luminosities and velocity dispersions are potentially very significant. A recent analysis of 7 strong lenses has been undertaken by Im et al. (1996). Their current results yield = 0.64^+0.15_-0.26 (i.e., this measurement sits almost at the end of the range excluded by Kochanek (1996) at 95% confidence. Im et al. exclude _m = 1.0 at 97% confidence.