A PROPOSAL FOR FINDING CLUSTERS OF GALAXIES AT Z-GREATER-THAN-OR-SIMILAR-TO-1

At large redshifts, a cluster or group may be too distant for the gala xies within the cluster to be detected individually in a short exposur e. However, the light from these ''undetected'' galaxies still modulat es the surface brightness of the background sky. Distant clusters can appear as 10 ''-1'.5 sized fluctuations in the surface brightness of t he extragalactic background light (EBL). The fluctuations have central surface brightnesses fainter than roughly 26 mag arcsec(-2) (in V) fo r clusters with z greater than or similar to 1 and are brighter than t he random fluctuations produced by held galaxies on comparable angular scales. While such low surface brightnesses features are difficult to detect with direct high-resolution imaging, we demonstrate that they are easily reached in short exposures through smoothing the sky in ver y flat CCD images.For a reasonable extrapolation of the properties and space densities of clusters and groups and for a wide range of cosmol ogical assumptions, we find that there should be tens of high-redshift dusters per square degree visible in the optical extragalactic backgr ound. Therefore, searching for small bright fluctuations in the EBL co uld be an efficient method for identifying high-redshift cluster candi dates. The detection rate is sensitive to the rate of cluster and gala xy evolution and may vary between 1 and 100 clusters per square degree . With follow-up imaging and spectroscopy of the cluster candidates, o ne can constrain both galaxy luminosity evolution and duster mass evol ution, and potentially discriminate between different cosmological mod els.