Substructure in Gravitational Lenses

In N-body simulations of CDM halo formation, several percent of the mass is left as satellites orbiting in their parent halo. This exceeds the mass fraction in observed Galactic satellites by at least an order of magnitude. Satellites orbiting a lens galaxy lead to "anomolous flux ratios," where two images should have the same flux ratio by symmetry but do not. We have developed a method to analyze the lens data to determine the average properties of the satellite population and find excellent agreement with the expectations for CDM.

(Left) In PG1115+080 the close image pair have had different fluxes ever since the system was discovered. The discrepancy cannot be explained by dust or microlensing. We analyzed 7 four-image systems to determine the amount of substructure in Dalal & Kochanek 2002a).

(Right) Here we superpose the resulting constraints on the predictions for standard CDM (heavy solid) line and several variants with massive neutrinos, tilts, or warm dark matter Dalal & Kochanek 2002b). Standard CDM matches our estimate of the amount of substructure perfectly.

Next we are looking for lenses with anomolous image astrometry, which will provide far better estimates of the masses of the satellites, the formation of extra images by the substructure, and its effects on extended VLBI jets.