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

TL;DR

This paper reevaluates the number density of line-of-sight haloes versus lens subhaloes in galaxy-galaxy strong lensing, showing previous estimates overstate the line-of-sight perturbers and emphasizing the importance of realistic detection criteria.

Contribution

It introduces a more accurate detection criterion for line-of-sight haloes in strong lensing, correcting overestimations in previous studies and providing refined estimates based on observational realism.

Findings

Line-of-sight perturbers are overestimated by up to a factor of two in previous work.

Deep imaging detects twice as many line-of-sight perturbers as subhaloes.

Moderate imaging depth slightly favors detection of line-of-sight perturbers over subhaloes.

Abstract

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. More in keeping with analyses of real-world observations, we regard a line-of-sight halo as detectable only if adding it to a smooth model generates a statistically significant improvement in the reconstructed image. We find that the number density of detectable line-of-sight perturbers has been overestimated by as much as a factor of two in the previous literature. For typical lensing geometries and configurations, very deep imaging is sensitive to twice as many line-of-sight perturbers as subhaloes, but moderate depth imaging is sensitive to only slightly more line-of-sight perturbers than subhaloes.