Completeness and Confusion in the Identification of Lyman-Break Galaxies

We have carried out a study to simulate distant clusters of galaxies in deep ground-based optical images. We find that when model galaxies are added to deep images obtained with the William Herschel Telescope, there is considerable scatter of the recovered galaxy colours away from the model values; this scatter is larger than that expected from photometric errors and is significantly affected by confusion, due to ground-based seeing, between objects in the field. In typical conditions of $\approx$ 1-arcsec seeing, the combination of confusion and incompleteness causes a considerable underestimation of the true surface density of $z \approx 3$ galaxies. We argue that the actual surface density of $z \approx 3$ galaxies may be several times greater than that estimated by previous ground-based studies, consistent with the surface density of such objects found in the HDF.