The galaxy population of the z = 1 cluster of galaxies MG 2016 + 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 +0.45 −0.57 and faint-end slope α = −0.60 −0.33, consistent with what is expected for a passively evolving population of galaxies formed at high redshift, zf > 2. From the total Ksband 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/LKs = 27 +64 −17 h50 (M/LKs)⊙, 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 colourmagnitude 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.