A photometric redshift analysis of galaxies in the field of the wide-separation gravitational lens MG2016+112 reveals a population of 69 galaxies with photometric redshifts consistent with being in a cluster at the redshift of the giant elliptical lensing galaxy z = 1.00 .
The Ks -band luminosity function of the cluster galaxies is well represented by the Schechter function with a characteristic magnitude K ^ { * } _ { s } = 18.90 _ { -0.57 } ^ { +0.45 } and faint-end slope \alpha = -0.60 ^ { +0.39 } _ { -0.33 } , consistent with what is expected for a passively evolving population of galaxies formed at high redshift , z _ { f } > 2 .
From the total Ks -band flux of the cluster galaxies and a dynamical estimate of the total mass of the cluster , the restframe Ks -band mass-to-light ratio of the cluster is derived to be M / L _ { Ks } = 27 ^ { +64 } _ { -17 } h _ { 50 } ( M / L _ { Ks } ) _ { \sun } , in agreement with the upper limit derived earlier from Chandra X-ray observations and the value derived locally in the Coma cluster .
The cluster galaxies span a red sequence with a considerable scatter in the colour-magnitude diagrams , suggesting that they contain young stellar populations in addition to the old populations of main-sequence stars that dominate the Ks -band luminosity function .
This is in agreement with spectroscopic observations which show that 5 out of the 6 galaxies in the field confirmed to be at the redshift of the lensing galaxy have emission lines .
The projected spatial distribution of the cluster galaxies is filamentary-like rather than centrally concentrated around the lensing galaxy , and show no apparent luminosity segregation .
A handful of the cluster galaxies show evidence of merging/interaction .
The results presented in this paper suggest that a young cluster of galaxies is assembling around MG2016+112 .