Weak lensing observations of the “ dark ” cluster MG 2016 + 112

We investigate the possible existence of a high-redshift (z = 1) cluster of galaxies associated with the QSO lens system MG2016+112. From an ultra-deep R-and less deep V-and I-band Keck images and a K-band mosaic from UKIRT, we detect ten galaxies with colors consistent with the lensing galaxy within 225h −1 kpc of the z = 1.01 lensing galaxy. This represents an overdensity of more than ten times the number density of galaxies with similar colors in the rest of the image. We also find a group of seven much fainter objects closely packed in a group only 27h −1 kpc northwest of the lensing galaxy. We perform a weak lensing analysis on faint galaxies in the R-band image and detect a mass peak of a size similar to the mass inferred from X-ray observations of the field, but located 64 ′′ northwest of the lensing galaxy. From the weak lensing data we rule out a similar sized mass peak centered on the lensing galaxy at the 2σ level.