2002ApJ...580..696M

When a gravitational lens produces two or more images of a quasar's radio jet the images can be compared to reveal the presence of small structures along one or more of the lines of sight. If mass is distributed smoothly on scales of ≲10⁷ M_☉, independent bends in the jet images on milliarcsecond scales will not be produced. All three of the well-collimated multiply imaged radio jets that have been mapped on milliarcsecond scales show some evidence of independent bends in their images. Using existing data, we model the lens system B1152+199 and show that it likely contains a substructure of mass ∼10⁵-10⁷ h^–1 M_☉or a velocity dispersion of ∼10 km.s^–1. An alternative explanation is that an intrinsic bend in the jet is undetected in one image and magnified in the other. This explanation is disfavored, and future observations could remove any ambiguity that remains. The probability of a radio jet being bent by small-scale structure both inside and outside of the host lens is then investigated. The known populations of dwarf galaxies and globular clusters are far too small to make this probability acceptable. A previously unknown population of massive dark objects is needed. The standard cold dark matter model might be able to account for the observations if small mass halos are sufficiently compact. In other cosmological models where small-scale structure is suppressed, such as standard warm dark matter, the observed bent jets would be very unlikely to occur.