Gravitational lensing with stochastic substructure: Effects of the clump mass function and spatial distribution
Mass clumps in gravitational lens galaxies can perturb lensed images in characteristic ways. Strong lens flux ratios have been used to constrain the amount of dark matter substructure in lens galaxies, and various other observables have been considered as additional probes of substructure. We study the general theory of lensing with stochastic substructure in order to understand how lensing observables depend on the mass function and spatial distribution of clumps. We find that magnification perturbations are mainly sensitive to the total mass in substructure projected near the lensed images; when the source is small, flux ratios are not very sensitive to the shape of the clump mass function. Position perturbations are mainly sensitive to a characteristic clump mass scale, namely m_eff = <m^2>/<m>, with some mild dependence on other mass moments when the spatial distribution is not uniform. They have contributions from both "local" and "global" populations of clumps (i.e., those projected near the images, and those farther away). Time delay perturbations are sensitive to the same characteristic mass, m_eff, and mainly driven by the global population of clumps. While there is significant scatter in all lensing quantities, there are some non-trivial correlations that may contain further information about the clump population. Our results indicate that a joint analysis of multiple lens observables will offer qualitatively new constraints on the mass function and spatial distribution of dark matter substructure in distant galaxies.