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 Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope. We have mapped fields in two massive, concentrated clusters, A370 at z = 0.37 and Cl 2244-02 at z = 0.33, at wavelengths of 450 and 850 mu m. The resulting continuum maps cover a total area of about 10 arcmin(2) to 1 sigma noise levels less than 14 and 2 mJy beam(-1) 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 (2.4 +/- 1.0) x 10(3) deg(-2) is found to a 50% completeness limit of similar to 4 mJy at 850 mu m. The submillimeter spectral properties of these sources indicate that the majority lie at high redshift, z > 1. Without correcting for lens 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 mu m 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.