Morphology and surface brightness evolution of z∼1.1 radio galaxies

We use K'-band (2.1-mu m) imaging to investigate the angular size and morphology of 10 6C radio galaxies, at redshifts 1 less than or equal to z less than or equal to 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 similar to 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 < 0.2. The 3CR galaxies at z similar to 1.1 show a comparable enhancement in surface brightness. The mean radius of the 6C galaxies suggests that they evolve into ellipticals of L similar to L* luminosity, and is consistent with their low-redshift counterparts being relatively small FRII galaxies similar to 25 times lower in radio luminosity, or small FRI galaxies similar to 1000 times lower in radio luminosity. Hence the 6C radio galaxies appear to undergo as much optical and radio evolution as the 3CR galaxies.