Faint radio samples: the key to understanding radio galaxies

The large number of differences between highand low-redshift radio galaxies have almost all been discovered by looking at the bright 3C sample of radio sources. This has the disadvantage that the strong correlation between radio luminosity and redshift within a single sample makes it impossible to be determine whether these differences are the result of cosmic evolution or whether they are simply the result of source properties depending on radio luminosity. The solution to this problem is to compare the properties of sources in faint samples with those of the 3C sources. I and collaborators have recently removed the degeneracy between redshift and radio luminosity by comparing the 3C sample to the recently completed 6C and 7C samples. In this paper I concentrate on what our study has revealed about the host galaxies of radio sources. At low redshift, radio galaxies are giant ellipticals with absolute magnitude being independent of radio luminosity over a range of 10 in radio luminosity. At z ∼ 1, the radio-luminous 3C radio galaxies are still giant ellipticals, but the 6C galaxies, only a factor of six lower in radio luminosity, are fainter by about 1 mag in the near-IR and have much more compact near-IR structures. At z ∼ 1, radio galaxies follow a line in a diagram of optical luminosity verses de Vaucouleurs scale length parallel to the projection of the fundamental plane for nearby ellipticals in this diagram. I discuss the significance of these results for our understanding of radio galaxies and