Gravitationally lensed arcs as galaxy redshift indicators

Abstract

For the observed giant arcs to be artifacts of the gravitational lens effect of clusters of galaxies on normal background galaxies, the galaxies must be behind the clusters. Since the redshifts of many of these clusters are known, they may be taken as the smallest possible redshifts for the background galaxies. These limits may give important redshift information for dim galaxies near the brightness detection limit of modern techniques. These limits may be in conflict with simple extrapolations of modern galaxy redshift surveys and some cold dark matter scenarios where galaxies form very late.