Discovery of a galaxy cluster in the foreground of the wide-separation quasar pair UM 425

We report the discovery of a cluster of galaxies in the field of UM 425, a pair of quasars separated by 6".5. Based on this finding, we revisit the long-standing question of whether this quasar pair is a binary quasar or a wide-separation lens. Previous work has shown that both quasars are at z = 1.465 and show broad absorption lines. No evidence for a lensing galaxy has been found between the quasars, but there are two hints of a foreground cluster: diffuse X-ray emission observed with Chandra, and an excess of faint galaxies observed with the Hubble Space Telescope. Here we show, via VLT spectroscopy, that there is a spike in the redshift histogram of galaxies at z = 0.77. We estimate the chance of finding a random velocity structure of such significance to be about 5%, and thereby interpret the diffuse X-ray emission as originating from z = 0.77 rather than from the quasar redshift. The mass of the cluster, as estimated from either the velocity dispersion of the z = 0.77 galaxies or the X-ray luminosity of the diffuse emission, would be consistent with the theoretical mass required for gravitational lensing. The positional offset between the X-ray centroid and the expected location of the mass centroid is ∼40 kpc, which is not too different from offsets observed in lower redshift clusters. However, UM 425 would be an unusual gravitational lens, by virtue of the absence of a bright primary lensing galaxy. Unless the mass-to-light ratio of the galaxy is at least 80 times larger than usual, the lensing hypothesis requires that the galaxy group or cluster plays a uniquely important role in producing the observed deflections.