SHEAR AND ELLIPTICITY IN GRAVITATIONAL LENSES

Galaxies modeled as singular isothermal ellipsoids with an axis ratio distribution similar to the observed axis ratio distribution of E and S0 galaxies are statistically consistent with both the observed number s of two-image and four-image lenses and the inferred ellipticities of individual lenses. However, no four-image lens is well fitted by the model (typical chi(2)/N-dof similar to 20), the axis ratio of the mode l can be significantly different from that of the observed lens galaxy , and the major axes of the model and the galaxy may be slightly misal igned. We found that models with a second, independent, external shear axis could fit the data well (typical chi(2)/N-dof similar to 1), whi le adding the same number of extra parameters to the radial mass distr ibution does not produce such a dramatic improvement in the fit. An in dependent shear axis can be produced by misalignments between the lumi nous galaxy and its dark matter halo, or by external shear perturbatio ns due to galaxies and clusters correlated with the primary lens or al ong the line of sight. We estimate that the external shear perturbatio ns have no significant effect on the expected numbers of two-image and four-image lenses but that they can be important perturbations in ind ividual lens models. However, the amplitudes of the external shears re quired to produce good fits are larger than our estimates for typical external shear perturbations (10%-15% shear instead of 1%-3%), suggest ing that the origin of the extra angular structure must be intrinsic t o the primary lens galaxy in most cases.