Spectroscopic confirmation of a cluster of galaxies at z = 1 in the field of the gravitational lens MG 2016-112

We present new optical data on the cluster AX J2019+1127 identified by the X-ray satellite ASCA at z ∼ 1 (Hattori et al. 1997). The data suggest the presence of a high-redshift cluster of galaxies responsible for the large separation triple quasar MG 2016+112. Our deep photometry reveals an excess of z ∼ 1 galaxy candidates, as already suspected by Benítez et al. (1999). Our spectroscopy survey of 44 objects in the field shows an excess of 6 red galaxies securely identified at z ∼ 1, with a mean redshift of z = 1.005 ± 0.002. We estimate a velocity dispersion of σ 771_-160⁴³⁰ km s^-1 based on these 6 galaxies and a V-band mass-to-light ratio of 215_-77⁺³⁰⁸h₅₀ (M/L_V)_⊙. Our observations thus confirm the existence of a massive structure acting as the lens, which explains the unusual configuration of the triple quasar. Hence, there is no need to invoke the existence of a "dark cluster" to understand this lens system.