Morphology and surface brightness evolution of z 1.1 radio galaxies

We use K′-band (2.1-µm) imaging to investigate the angular size and morphology of 10 6C radio galaxies, at redshifts 1≤z≤1.4. Two radio galaxies appear to be undergoing mergers, another contains, within a single envelope, two intensity peaks aligned with the radio jets, while the other seven appear consistent with being normal ellipticals in the K band. Intrinsic half-light radii are estimated from the areas of each radio galaxy image above a series of thresholds. The 6C galaxy radii are found to be significantly smaller than those of the more radio-luminous 3CR galaxies at similar redshifts. This would indicate that the higher mean K-band luminosity of the 3CR galaxies reflects a difference in the size of the host galaxies, and not solely a difference in the power of the active nuclei. The size-luminosity relation of the z∼1.1 6C galaxies indicates a 1.0–1.6 mag enhancement of their rest frame R-band surface brightness relative to either local ellipticals of the same size or FRII radio galaxies at z