Galaxies and galaxy groups located along the line of sight towards gravitationally lensed quasars produce high-order perturbations of the gravitational potential at the lens position .
When these perturbation are too large , they can induce a systematic error on H _ { 0 } of a few-percent if the lens system is used for cosmological inference and the perturbers are not explicitly accounted for in the lens model .
In this work , we present a detailed characterization of the environment of the lens system WFI 2033 - 4723 ( z _ { src } = 1.662 , z _ { lens } = 0.6575 ) , one of the core targets of the H0LiCOW project for which we present cosmological inferences in a companion paper ( Rusu et al .
2019 ) .
We use the Gemini and ESO-Very Large telescopes to measure the spectroscopic redshifts of the brightest galaxies towards the lens , and use the ESO-MUSE integral field spectrograph to measure the velocity-dispersion of the lens ( \sigma _ { los } = 250 ^ { +15 } _ { -21 } km s ^ { -1 } ) and of several nearby galaxies .
In addition , we measure photometric redshifts and stellar masses of all galaxies down to i < 23 mag , mainly based on Dark Energy Survey imaging ( DR1 ) .
Our new catalog , complemented with literature data , more than doubles the number of known galaxy spectroscopic redshifts in the direct vicinity of the lens , expanding to 116 ( 64 ) the number of spectroscopic redshifts for galaxies separated by less than 3 \arcmin ( 2 \arcmin ) from the lens .
Using the flexion-shift as a measure of the amplitude of the gravitational perturbation , we identify 2 galaxy groups and 3 galaxies that require specific attention in the lens models .
The ESO MUSE data enable us to measure the velocity-dispersions of three of these galaxies .
These results are essential for the cosmological inference analysis presented in Rusu et al .
( 2019 ) .