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 C_in 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.