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

Understand

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.