Deep optical imaging of the field of PC 1643+4631 A&B - I. Spatial distributions and the counts of faint galaxies

Abstract

We present deep optical images of the PC 1643+4631 field, obtained at the William Herschel Telescope (WHT). This field contains two quasars at redshifts z = 3.79 and 3.83 and a cosmic microwave background (CMB) decrement detected with the Ryle Telescope. The images are in U, G, V, R and I filters, and in two custom-built narrow-band filters. Using the object-finding algorithms FOCAS and SEXTRACTOR, we have constructed galaxy catalogues in each band. The catalogues created with the two detection algorithms are compared to determine the faint magnitude limits to which they are consistent. Using Monte Carlo simulations, we investigate the completeness of the catalogues at faint magnitudes. In particular, we compare the flux lost using isophotal apertures on a real image with that on a noise-only image: recovery of artificial galaxies from the noise-only image significantly overestimates the flux lost from the galaxies, and we find that the corrections made using this technique suffer a systematic error of some 0.4 mag. We determine that the catalogues are complete to RAB, GAB = 25.0 and to UAB = 25.5, and we investigate the number counts and spatial distributions of the galaxies in the field. The galaxy counts in each band are consistent with the results of other recent deep imaging surveys. We investigate the properties of several populations of objects in the field. Our narrow-band imaging does not reveal the presence of any bright emission-line galaxies, nor do we detect any obvious candidate for a further, possibly lensed, quasar image. The galaxies whose colours indicate that they lie at redshifts 1 ≲ z ≲ 2 are found to have similar properties as in other fields. However, we find that the number of high-redshift Lyman-break candidate galaxies in the field is significantly higher than would be expected from the mean surface density found in the surveys of Steidel et al. The Lyman-break candidates are strongly clustered, but they are not concentrated towards the CMB decrement; rather, they tend to lie towards the edges of the field. Finally, using the present data in combination with existing near-infrared imaging of the field, we investigate extremely red objects (EROs) in the field. We find a large surface density of EROs, and we find that their colours are consistent with those of slightly-reddened early-type galaxies at redshift z ≈ 2.