MORPHOLOGY AND SURFACE BRIGHTNESS EVOLUTION OF Z-SIMILAR-TO-1.1 RADIOGALAXIES

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 underg oing mergers, another contains, within a single envelope, two intensit y peaks aligned with the radio jets, while the other seven appear cons istent with being normal ellipticals in the K band. Intrinsic half-lig ht radii are estimated from the areas of each radio galaxy image above a series of thresholds. The 6C galaxy radii are found to be significa ntly smaller than those of the more radio-luminous 3CR galaxies at sim ilar redshifts. This would indicate that the higher mean K-band lumino sity 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 nucl ei. The size-luminosity relation of the z similar to 1.1 6C galaxies i ndicates 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 FR II radio galaxies at z < 0.2. The 3CR galaxies at z similar to 1.1 sho w a comparable enhancement in surface brightness. The mean radius of t he 6C galaxies suggests that they evolve into ellipticals of L similar to L luminosity, and is consistent with their low-redshift counterpa rts 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 mu ch optical and radio evolution as the 3CR galaxies.