The evolution of 3CR radio galaxies from z = 1

We present the results of a comprehensive re-analysis of the images of a vi rtually complete sample of 28 powerful 3CR radio galaxies with redshifts 0. 6 < z < 1.8 from the Hubble Space Telescope (HST) archive. Using a two-dime nsional modelling technique we have derived scalelengths and absolute magni tudes for a total of 16 3CR galaxies with a median redshift of z=0.8. Our r esults confirm the basic conclusions of Best, Longair & Rottgering in that we also find z=1 3CR galaxies to be massive, well-evolved ellipticals, the infrared emission of which is dominated by starlight. However, we in fact f ind that the scalelength distribution of 3CR galaxies at z similar or equal to 1 is completely indistinguishable from that derived for their low-redsh ift counterparts from our own recently completed HST study of active galact ic nuclei hosts at z similar or equal to 0.2. There is thus no evidence tha t 3CR radio galaxies at z similar or equal to 1 are dynamically different f rom 3CR galaxies at low redshift. Moreover, for a 10-object subsample we ha ve determined the galaxy parameters with sufficient accuracy to demonstrate , for the first time, that the z similar or equal to 1 3CR galaxies follow a Kormendy relation that is indistinguishable from that displayed by low-re dshift ellipticals if one allows for purely passive evolution. The implied rather modest level of passive evolution since z similar or equal to 1 is c onsistent with that predicted from spectrophotometric models provided one a ssumes a high formation redshift (z greater than or equal to 4) within a lo w-density universe. We conclude that there is no convincing evidence for si gnificant dynamical evolution among 3CR galaxies in the redshift interval 0 < z < 1, and that simple passive evolution remains an acceptable interpret ation of the K-z relation for powerful radio galaxies.