The Evolution of 3CR Radio Galaxies from z=1

We present the results of a comprehensive re-analysis of the images of a virtually complete sample of 28 powerful 3CR radio galaxies with redshifts 0.6<z<1.8 from the HST archive. Using a two-dimensional modelling technique we have derived scalelengths and absolute magnitudes for a total of 16 3CR galaxies with a median redshift of z=0.8. Our results confirm the basic conclusions of Best, Longair & R\"{o}ttgering (1997, 1998) in that we also find z=1 3CR galaxies to be massive, well-evolved ellipticals, whose infrared emission is dominated by starlight. However, we in fact find that the scalelength distribution of 3CR galaxies at z \simeq 1 is completely indistinguishable from that derived for their low-redshift counterparts from our own recently-completed HST study of AGN hosts at z \simeq 0.2. There is thus no evidence that 3CR radio galaxies at z \simeq 1 are dynamically different from 3CR galaxies at low redshift. Moreover, for a 10-object sub-sample we have determined the galaxy parameters with sufficient accuracy to demonstrate, for the first time, that the z \simeq 1 3CR galaxies follow a Kormendy relation which is indistinguishable from that displayed by low-redshift ellipticals if one allows for purely passive evolution. The implied rather modest level of passive evolution since z \simeq 1 is consistent with that predicted from spectrophotometric models provided one assumes a high formation redshift (z \ge 4) within a low-density Universe. We conclude that there is no convincing evidence for significant dynamical evolution among 3CR galaxies in the redshift interval 0<z<1, and that simple passive evolution remains an acceptable interpretation of the K-z relation for powerful radio galaxies.