Exploring Effects on Magnifications due to Line-of-Sight Galaxies in the Hubble Frontier Fields

TL;DR

This study develops three-dimensional mass models for the Hubble Frontier Fields to assess how line-of-sight galaxies affect gravitational lensing magnifications and image positions, revealing biases in traditional two-dimensional models.

Contribution

It introduces the first full 3D mass models for these fields and evaluates the systematic biases caused by neglecting line-of-sight galaxies in lensing analyses.

Findings

Omitting line-of-sight galaxies causes image position offsets of 0.1-0.4".

Median magnification shifts up to 6% due to line-of-sight effects.

Approximate 5% magnification bias persists even with simplified models.

Abstract

Cluster lensing has become an important tool in the search for high redshift galaxies through its ability to magnify sources. In order to determine the intrinsic properties of these galaxies, lensing mass models must be constructed to determine the magnification of the images. These models are traditionally two-dimensional, focusing on the mass within the cluster and either ignoring or approximating any contribution from line-of-sight galaxies. In this paper, we present the first full set of three-dimensional mass models of the six Hubble Frontier Fields and use them to test for systematic biases in magnifications due to using the traditional 2D approach. We find that omitting foreground or background galaxies causes image position offsets between 0.1-0.4", a non-negligible fraction of the typical 0.3-0.7" residuals of current state-of-the-art models. We also find that median image magnifications can shift by up to 6%, though it is dependent on the field. This can be alleviated in some cases by approximating the mass in the lensing plane, but a 5% magnification bias still exists in other cases; image position offsets are also improved, but are still present at 0.10".