Strong Lensing by Subhalos in the Dwarf-Galaxy Mass Range II: Detection
  Probabilities
  The dark halo substructures predicted by current cold dark matter simulations
may in principle be detectable through strong-lensing image splitting of
quasars on small angular scales (0.01 arcseconds or below). Here, we estimate
the overall probabilities for lensing by substructures in a host halo closely
aligned to the line of sight to a background quasar. Under the assumption that
the quasar can be approximated as a point source, the optical depth for strong
gravitational lensing by subhalos typically turns out to be very small (tau <
0.01), contrary to previous claims. We therefore conclude that it is currently
not feasible to use this strategy to put the simulation predictions for the
dark matter subhalo population to the test. However, if one assumes the source
to be spatially extended, as is the case for a quasar observed at radio
wavelengths, there is a reasonable probability for witnessing substructure
lensing effects even at rather large projected distances from the host galaxy,
provided that the angular resolution is sufficient. While multiply-imaged,
radio-loud quasars would be the best targets for unambiguously detecting dark
matter subhalos, even singly-imaged radio quasars might be useful for setting
upper limits on the abundance and central surface mass density of subhalos.
