The galaxy population of the z=1 cluster of galaxies MG2016+112

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(-0.33)(+0.39), 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(-17)(+64) h(50)(M/L-Ks)(circle dot), 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.