Simulations of Strong Gravitational Lensing with Substructure

Galactic sized gravitational lenses are simulated by combining a cosmological N-body simulation and models for the baryonic component of the galaxy. The lens caustics, critical curves, image locations and magnification ratios are calculated by ray-shooting on an adaptive grid. It is found that the simulations do not cause the observed number of violations of the cusp caustic magnification relation. A part of this may be due to an insufficient amount of substructure but even for unsmoothed simulations (with maximal substructure) the factor of three increase in the number of violations still did not match the data. This suggests that other factors play an important role. These may include lensing by structure outside the halo, selection bias and the possibility that a randomly selected galaxy halo may be more irregular, for example due to recent mergers, than the isolated halo used in this study. It is also shown that, with the computed level of substructure, the image magnifications of the Einstein cross type lenses are very weak functions of source size up to ∼ 1 kpc. This is also true for the magnification ratios of widely separated images in the fold and cusp caustic lenses. This means that the magnification ratios for all the emission regions of a lensed quasar should agree with each other, barring microlensing by stars. The source size dependence of the magnification ratio between the closest pair of images is more sensitive to substructure. The ratios of image multiplicities are calculated. The ratio of four image to two image lenses is found to be ∼ 8% for our elliptical galaxies and ∼ 40% for our disk galaxies where there is also a ∼ 6% chance of having a three image, naked cusp, lens. Subject headings: gravitational lensing – cosmology: dark matter – galaxy:structure