Deep optical imaging of the field of PC1643+4631A&B, II: Estimating the   colours and redshifts of faint galaxies
  In an investigation of the cause of the cosmic microwave background decrement in the field of the z = 3.8 quasar pair PC1643+4631, we have carried out a study to photometrically estimate the redshifts of galaxies in deep multi-colour optical images of the field taken with the WHT. To examine the possibility that a massive cluster of galaxies lies in the field, we have attempted to recover simulated galaxies with intrinsic colours matching those of the model galaxies used in the photometric redshift estimation. We find that when such model galaxies are added to our images, there is considerable scatter of the recovered galaxy redshifts away from the model value; this scatter is larger than that expected from photometric errors and is the result of confusion, simply due to ground-based seeing, between objects in the field. We have also compared the likely efficiency of the photometric redshift technique against the colour criteria used to select z>3 galaxies via the strong colour signature of the Lyman-limit break. We find that these techniques may significantly underestimate the true surface density of z>3, due to confusion between the high-redshift galaxies and other objects near the line of sight. We argue that the actual surface density of z=3 galaxies may be as much as 6 times greater than that estimated by previous ground-based studies, and note that this conclusion is consistent with the surface density of high-redshift objects found in the HDF. Finally, we conclude that all ground-based deep field surveys are inevitably affected by confusion, and note that reducing the effective seeing in ground-based images will be of paramount importance in observing the distant universe. 
Garret Cotter, Toby Haynes, Joanne C. Baker, Michael E. Jones, Richard   Saunders