Galaxy-galaxy strong lens perturbations: line-of-sight haloes versus lens subhaloes

Understand

We rederive the number density of intervening line-of-sight haloes relative to lens subhaloes in galaxy-galaxy strong lensing observations, where these perturbers can generate detectable image fluctuations.

• Previous studies have calculated the detection limit of a line-of-sight small-mass dark halo by comparing the lensing deflection angles it would cause, to those caused by a subhalo within the lens.
• However, this overly simplifies the difference in observational consequences between a subhalo and a line-of-sight halo.
• Furthermore, it does not take into account degeneracies between an extra subhalo and the uncertain properties of the main lens.