2003MNRAS.344..337T

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^+0.45_–0.57and faint-end slope α=-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 rest-frame Ks -band mass-to-light ratio of the cluster is derived to be M/L_Ks= 27⁺⁶⁴_–17h₅₀(M/L_Ks)_☉, 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 shows 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.