Abstract
ABSTRACT Measurements of the Hubble–Lemaitre constant from early- and local-Universe observations show a significant discrepancy. In an attempt to understand the origin of this mismatch, independent techniques to measure H0 are required. One such technique, strong lensing time delays, is set to become a leading contender amongst the myriad methods due to forthcoming large strong lens samples. It is therefore critical to understand the systematic effects inherent in this method. In this paper, we quantify the influence of additional structures along the line of sight by adopting realistic light-cones derived from the cosmoDC2 semi-analytical extragalactic catalogue. Using multiple-lens plane ray tracing to create a set of simulated strong lensing systems, we have investigated the impact of line-of-sight structures on time-delay measurements and in turn, on the inferred value of H0. We have also tested the reliability of existing procedures for correcting for line-of-sight effects. We find that if the integrated contribution of the line-of-sight structures is close to a uniform mass sheet, the bias in H0 can be adequately corrected by including a constant external convergence κext in the lens model. However, for realistic line-of-sight structures comprising many galaxies at different redshifts, this simple correction overestimates the bias by an amount that depends linearly on the median external convergence. We therefore conclude that lens modelling must incorporate multiple-lens planes to account for line-of-sight structures for accurate and precise inference of H0.
Highlights
The Hubble–Lemaitre constant, H0, is a cornerstone of the standard cosmological model, setting the distance scale, age, and critical density of the Universe
To quantify the influence of secondary deflectors on the measurement of H0 with strong lensing time delays, we have simulated approximately 800 galaxy-scale strong lensing systems with quadruply lensed variable point sources; half of these were created with a primary lens and line-of-sight haloes and half with the same primary lens plus a constant external convergence and shear
Our main conclusion is that incorporating constant external convergence in the modelling only works reliably if the lensed time delays are subjected to a uniform external convergence
Summary
The Hubble–Lemaitre constant, H0, is a cornerstone of the standard cosmological model, setting the distance scale, age, and critical density of the Universe. Independent from any of the aforementioned methods, strong lensing time delays provide valuable measurements of H0 (e.g. Shajib et al 2020; Wong et al 2020) that may assist in the understanding of these discrepancies once systematic uncertainties in the technique are fully calibrated With such systematics in mind, in this paper we focus on the effects of line-of-sight structure, one of the most dominant sources of error in the lens time-delay method. Rim cosmological N-body simulation (Heitmann et al 2019) By modelling these time delays with the same methods used for real data, we directly assess the biases introduced by line-of-sight effects and the efficacy with which these can be accounted for using external corrections such as γ ext and κext.
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