Near-infrared imaging of the proposed z=2 cluster behind CL 0939+4713 (Abell 851)

We have obtained deep J and K' images of a 90-arcsec square field containing the z = 2.0 QSO that is viewed through the z = 0.4 rich cluster A851. Brightnesses of individual galaxies are measured from these data, and archival F702W WFPC2 images from HST. The results are used to construct two-color diagrams, color-magnitude diagrams, and luminosity functions of galaxies in A851 and the faint objects discovered by Dressier et al. (1993, AQJ, 404, L45) near the QSO. The QSO faint companions appear to form a separate population from other faint galaxies in the field. Comparisons with the GISSEL models covering redshifts from 0 to 3 indicate that (1) the A851 galaxies have ages between 3 and 10 Gyr, and (2) if they are at the QSO redshift, the QSO companion galaxies are within 1 Gyr of cessation of star formation.