TDCOSMO

XIV. Practical techniques for estimating external convergence of strong gravitational lens systems and applications to the SDSS J0924+0219 system

¹ Department of Physics and Astronomy, University of California, 95603 Davis, CA, USA
e-mail: pwells@ucdavis.edu
² National Astronomical Observatory of Japan, 181-8588 Tokyo, Japan

Received: 6 February 2023
Accepted: 29 May 2023

Abstract

Context. Time-delay cosmography uses strong gravitational lensing of a time-variable source to infer the Hubble constant. The measurement is independent from both traditional distance ladder and CMB measurements. An accurate measurement with this technique requires considering the effects of objects along the line of sight outside the primary lens, which is quantified by the external convergence (κ_ext). In absence of such corrections, H₀ will be biased towards higher values in overdense fields and lower values in underdense fields.

Aims. We discuss the current state of the methods used to account for environment effects. We present a new software package built for this kind of analysis and others that can leverage large astronomical survey datasets. We apply these techniques to the SDSS J0924+0219 strong lens field.

Methods. We infer the relative density of the SDSS J0924+0219 field by computing weighted number counts for all galaxies in the field, and comparing to weighted number counts computed for a large number of fields in a reference survey. We then compute weighted number counts in the Millennium Simulation and compare these results to infer the external convergence of the lens field.

Results. Our results show the SDSS J0924+0219 field is a fairly typical line of sight, with median κ_ext = −0.012 and standard deviation σ_κ = 0.028.