  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 <~
10^7 Msun independent bends in the jet images on milli-arcsecond scales will
not be produced. All three of the well collimated multiply imaged radio jets
that have been mapped on milli-arcsecond 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 ~10^5 - 10^7
h^-1 Msun or a velocity dispersion of ~ 10 km/s. 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 investigate.
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 (CDM) 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 (WDM), the observed bent jets would be very unlikely
to occur.
