Shear and Ellipticity in Gravitational Lenses

Galaxies modeled as singular isothermal ellipsoids with an axis ratiodistribution similar to the observed axis ratio distribution of E and S0galaxies are statistically consistent with both the observed numbers oftwo-image and four-image lenses and the inferred ellipticities of individuallenses. However, no four-image lens is well fit by the model (typical$\chi^2/N_{dof} \sim 20$), the axis ratio of the model can be significantlydifferent from that of the observed lens galaxy, and the major axes of themodel and the galaxy may be slightly misaligned. We found that models with asecond, independent, external shear axis could fit the data well (typical$\chi^2/N_{dof} \sim 1$), while adding the same number of extra parameters tothe radial mass distribution does not produce such a dramatic improvement inthe fit. An independent shear axis can be produced by misalignments between theluminous galaxy and its dark matter halo, or by external shear perturbationsdue to galaxies and clusters correlated with the primary lens or along the lineof sight. We estimate that the external shear perturbations have no significanteffect on the expected numbers of two-image and four-image lenses, but thatthey can be important perturbations in individual lens models. However, theamplitudes of the external shears required to produce the good fits are largerthan our estimates for typical external shear perturbations (10-15% shearinstead of 1-3% shear) suggesting that the origin of the extra angularstructure must be intrinsic to the primary lens galaxy in most cases.Comment: 38 pages, 9 figures, submitted to Ap