H0LiCOW II. Spectroscopic survey and galaxy-group identification of the strong gravitational lens system HE0435-1223

TL;DR

This study conducts a detailed spectroscopic survey around the gravitational lens HE0435-1223, identifying galaxy groups and assessing their impact on lensing measurements crucial for determining the Hubble constant.

Contribution

It provides a comprehensive galaxy redshift catalog near HE0435-1223 and evaluates the influence of nearby galaxies and groups on lens modeling and H0 measurement.

Findings

Confirmed the lens is part of a small galaxy group.

Identified 8 additional galaxy group candidates near the line of sight.

Determined that only a few bright galaxies significantly perturb the lens potential.

Abstract

Galaxies located in the environment or on the line of sight towards gravitational lenses can significantly affect lensing observables, and can lead to systematic errors on the measurement of $H_0$ from the time-delay technique. We present the results of a systematic spectroscopic identification of the galaxies in the field of view of the lensed quasar HE0435-1223, using the W. M. Keck, Gemini and ESO-Very Large telescopes. Our new catalog triples the number of known galaxy redshifts in the vicinity of the lens, expanding to 102 the number of measured redshifts for galaxies separated by less than 3 arcmin from the lens. We complement our catalog with literature data to gather redshifts up to 15 arcmin from the lens, and search for galaxy groups or clusters projected towards HE0435-1223. We confirm that the lens is a member of a small group that includes at least 12 galaxies, and find 8 other group candidates near the line of sight of the lens. The flexion shift, namely the shift of lensed images produced by high order perturbation of the lens potential, is calculated for each galaxy/group and used to identify which objects produce the largest perturbation of the lens potential. This analysis demonstrates that i) at most three of the five brightest galaxies projected within 12 arcsec of the lens need to be explicitly used in the lens models, and ii) the groups can be treated in the lens model as an external tidal field (shear) contribution.