The Detection of Pure Dark Matter Objects with Bent Multiply Imaged Radio Jets

When a gravitational lens produces two or more images of a quasar's radio jetthe images can be compared to reveal the presence of small structures along oneor more of the lines of sight. If mass is distributed smoothly on scales of <~10^7 Msun independent bends in the jet images on milli-arcsecond scales willnot be produced. All three of the well collimated multiply imaged radio jetsthat have been mapped on milli-arcsecond scales show some evidence ofindependent bends in their images. Using existing data we model the lens systemB1152+199 and show that it likely contains a substructure of mass ~10^5 - 10^7h^-1 Msun or a velocity dispersion of ~ 10 km/s. An alternative explanation isthat an intrinsic bend in the jet is undetected in one image and magnified inthe other. This explanation is disfavored and future observations could removeany ambiguity that remains. The probability of a radio jet being bent by smallscale structure both inside and outside of the host lens is then investigate.The known populations of dwarf galaxies and globular clusters are far too smallto make this probability acceptable. A previously unknown population of massivedark objects is needed. The standard Cold Dark Matter (CDM) model might be ableto account for the observations if small mass halos are sufficiently compact.In other cosmological models where small scale structure is suppressed, such asstandard Warm Dark Matter (WDM), the observed bent jets would be very unlikelyto occur.Comment: Significantly improved and extended - Better comparison with observations and a more complete treatment of alternative explanations that do not require substructure. Some small errors are corrected. 16 pages, 4 figures, submitted to Ap