Subaru first-light deep photometry of galaxies in A 851 field

We report on the results of optical and near-infrared photometry of the cluster of galaxies A 851 (= CL 0939+4713) carried out as one of the first-light observations of the 8.2 m Subaru telescope on Mauna Kea. Images as sharp as 0.″3 FWHM in K′ and 0.″45 in R were obtained in these observations. The 3 σ limiting magnitudes for points sources in 0.″3 seeing with a 0.″'6 software aperture were shown to go down to R = 28.1, J = 25.1. and K′ = 24.0, respectively, for about 1 hour integration. Subaru photometry of the galaxies in this cluster has shown in its color-magnitude diagrams a well-defined sequence of the early-type galaxy population that is consistent with the track predicted for a single-burst passive-evolution model of galaxies at z = 0.4. We attempted a morphological classification of galaxies in the Subaru R-band image using the Cin method, and found that discrimination between ellipticals and spirals can be achieved fairly consistently with the types assigned by MORPHs on the HST F702W image down to R < 23. A weak lensing analysis made on the Subaru R-band image yielded a reconstructed surface mass-density distribution that shows a significant maximum corresponding to the peak of the smoothed luminosity distribution of cluster galaxies. We found no significant excess of faint and/or small galaxies in the putative cluster area around the z = 2 quasar and no difference in the colors for galaxies in this area from those in the main cluster either. Two extremely red objects (ERO) with R - K > 6 are newly identified in the field. The colors of the reddest, disk-shaped galaxy R1 are found compatible with those of an unreddened E/S0 galaxy at z = 1.6.