Imaging of the field of 4C 41.17 below the Lyman limit

Imaging of z greater than or equal to 3.4 radio galaxy fields below the Lyman continuum wavelength allows companion galaxies to be identified on the basis of red colours across the wavelength of redshifted Ly alpha and very red colours across the redshifted Lyman continuum. These arise owing to a combination of absorption by intervening Ly alpha forest and Lyman-limit systems, and intrinsic Lyman-limit breaks in the galaxy spectral energy distribution caused by an H I screen or breaks in stellar spectra. As a pilot study we have imaged the field of the z = 3.8 radio galaxy 4C 41.17 in U, V and R with the Auxiliary Port of the William Herschel Telescope (WHT). We find a number of potential companion galaxies, which require confirmation via spectroscopy or narrow-band imaging. The Lyman limit in the spectrum of the radio galaxy itself and its implications for the origin of the UV flux are also discussed.