A Deep Submillimeter Survey of Lensing Clusters: a New Window on Galaxy Formation and Evolution

We present the first results of a submillimeter survey of distant clusters using the new Submillimeter CommonUser Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope. We have mapped fields in two massive, concentrated clusters, A370 at and Cl 2244202 at , at wavelengths of 450 and 850 mm. The z 5 0.37 z 5 0.33 resulting continuum maps cover a total area of about 10 arcmin to 1 j noise levels less than 14 and 2 mJy beam at the two wavelengths, 2–3 orders of magnitude deeper than was previously possible. We have concentrated on lensing clusters to exploit the amplification of all background sources by the cluster, improving the sensitivity by a factor of 1.3–2 as compared with a blank-field survey. A cumulative source surface density of deg is found to a 50% completeness limit of ∼4 mJy at 850 mm. The submillimeter spectral 3 (2.4 5 1.0) # 10 properties of these sources indicate that the majority lie at high redshift, . Without correcting for lens z 1 1 amplification, our observations limit the blank-field counts at this depth. The surface density is 3 orders of magnitude greater than the expectation of a nonevolving model using the local IRAS 60 mm luminosity function. The observed source counts thus require a substantial increase in the number density of strongly star-forming galaxies in the high-redshift universe and suggest that optical surveys may have substantially underestimated the star formation density in the distant universe. Deeper submillimeter surveys with SCUBA should detect large numbers of star-forming galaxies at high redshift and so provide strong constraints on the formation of normal galaxies. Subject headings: cosmology: observations — early universe — galaxies: evolution — galaxies: formation — gravitational lensing — radio continuum: galaxies