AN OPTICALLY LUMINOUS RADIO GALAXY AT Z = 3.22 AND THE K-Z DIAGRAM ATHIGH-REDSHIFT

We present images of the z = 3.22 radio galaxy 6C 1232 + 39 taken thro ugh the B, R, I, J, H, and K filters, an image taken through a narrow- band filter centered on the Lyalpha line, and an image made with the V LA at 5 GHz. Unlike the two other z > 3 radio galaxies that have been extensively studied (B2 0902 + 34 and 4C 41.17), this radio galaxy has a ''classical double '' radio structure. The structure of the radio g alaxy is a strong function of wavelength, changing from a long linear structure at the shortest wavelength (B) to one that is dominated by a single compact component at the longest wavelength (K). The galaxy's optical, emission-line, and radio structures are closely aligned; apar t from the overall alignment, there is, however, not the clear corresp ondence between the continuum and Lyalpha structures that would be exp ected if both trace the same young starburst regions. We show that, li ke the two other well-studied z > 3 radio galaxies, 6C 1232 + 39 has a red bump in its rest-frame optical spectrum. Low scatter in the K-z d iagram is often cited as evidence that radio galaxies formed early in the history of the universe. We show that there is now some evidence t hat the scatter in the K-z diagram at z > 2 is greater than the scatte r at z < 2. We discuss briefly the possible causes of this increased s catter, noting that near-infrared spectroscopy will be crucial in dete rmining whether the increased scatter gives any useful information abo ut the epoch at which radio galaxies formed. Finally we argue that the dispersion in the K-band magnitudes of the known z > 2 radio galaxies , and in particular the existence of luminous objects like 6C 1232 + 3 9, calls into question the use of the K-z Hubble relation to estimate the redshifts of distant radio galaxies from K magnitudes alone, a met hod that underpins the only published evidence for a redshift cutoff f or radio galaxies.