[Paper Review] Galaxy-galaxy strong lens perturbations: line-of-sight haloes versus lens subhaloes

Qiuhan He, Ran Li+9 more|arXiv (Cornell University)|Oct 9, 2021

Galaxies: Formation, Evolution, Phenomena64 references20 citations

TL;DR

This study re-evaluates the detectability of line-of-sight haloes versus lens subhaloes in galaxy-galaxy strong lensing by simulating realistic mock images and assessing statistical significance of image reconstruction improvements. It finds that previous estimates overestimated line-of-sight perturber detection by up to a factor of two, with moderate-depth imaging sensitive to only slightly more line-of-sight haloes than subhaloes, while deep imaging detects roughly twice as many line-of-sight perturbers as 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.

Motivation & Objective

• To re-evaluate the relative number density of detectable line-of-sight haloes compared to lens subhaloes in galaxy-galaxy strong lensing.
• To address limitations in prior studies that relied on deflection angle comparisons and effective mass approximations.
• To assess detectability using a statistically rigorous framework based on image reconstruction improvement, accounting for noise and lens model degeneracies.
• To quantify how exposure depth affects the sensitivity to low-mass perturbers and the relative contribution of line-of-sight haloes versus subhaloes.
• To provide a more accurate basis for constraining dark matter models using strong lensing observations.

Proposed method

• Simulated realistic strong lensing mock images using high-resolution hydrodynamics simulations including baryonic physics and full lens modeling.
• Assessed detectability by testing whether adding a line-of-sight halo to a smooth lens model produces a statistically significant improvement in image reconstruction.
• Used Bayesian inference with PyAutoFit and PyAutoLens to model lens systems and quantify detection thresholds.
• Computed detection limits (mth) for line-of-sight NFW haloes at various redshifts and exposure times (2000s to 8000s).
• Compared results to prior works (Li17, D18) using deflection angle equivalence and found systematic overestimation of line-of-sight halo detectability.
• Analyzed sensitivity functions and mass-threshold-redshift relations across multiple lens configurations and redshifts.

Experimental results

Research questions

• RQ1 How does the detectability of line-of-sight haloes compare to that of lens subhaloes when assessed through statistical image reconstruction improvement?
• RQ2 To what extent were previous estimates of line-of-sight halo number density overestimated due to simplified deflection angle comparisons?
• RQ3 How does exposure time affect the sensitivity to low-mass line-of-sight perturbers and subhaloes?
• RQ4 How do lensing geometry and source redshift influence the relative detectability of line-of-sight haloes versus subhaloes?
• RQ5 What is the impact of mass-concentration scatter and halo clustering on the detectability of low-mass perturbers?

Key findings

• The number density of detectable line-of-sight perturbers has been overestimated by up to a factor of two in previous literature due to reliance on deflection angle equivalence.
• For typical lensing configurations, moderate-depth imaging is sensitive to only slightly more line-of-sight perturbers than subhaloes, contrary to earlier claims of a 3–10× dominance.
• Very deep imaging (8000s exposure) is sensitive to approximately twice as many line-of-sight perturbers as subhaloes, indicating a strong dependence on depth.
• The detection threshold for line-of-sight haloes varies with redshift, with the lowest thresholds found at intermediate redshifts (z ≈ 0.5–0.7) for a lens at z = 0.5.
• The sensitivity function shows that perturbers at higher redshifts (z > 0.5) are harder to detect due to reduced angular scale and increased noise.
• The study confirms that mass-concentration scatter can boost detectability, but this effect is not sufficient to offset the overestimation from prior methods.