Gravitational lensing by elliptical galaxies

The fraction of high-redshift sources which are multiply imaged by interven ing galaxies is strongly dependent on the cosmological constant, and so can be a useful probe of the cosmological model. However its power is limited by various systematic (and random) uncertainties in the calculation of lens ing probabilities, one of the most important of which is the dynamical norm alization of elliptical galaxies. Assuming ellipticals' mass distributions can be modelled as isothermal spheres, the mass normalization depends on th e velocity anisotropy, the luminosity density, the core radius and the area over which the velocity dispersion is measured. The differences in the len sing probability and optical depth produced by using the correct normalizat ion can be comparable to the differences between even the most extreme cosm ological models. The existing data are not sufficient to determine the corr ect normalization with enough certainty to allow lensing statistics to be u sed to their full potential. However, as the correct lensing probability is almost certainly higher than is usually assumed, upper bounds on the cosmo logical constant are not weakened by these possibilities.