The evolution of radio galaxies
Lilly, Simon John
Investigations into several aspects of the evolution of the massive elliptical galaxies associated with powerful radio sources are described. Many of the galaxies studied are amongst the most distant stellar systems presently known and observations of these allow detailed comparisons of their properties to be made over timescales that are greater than a half the present age of the Universe.New redshifts have been measured for eight 3C radio source identifications. Several of these are very faint radio galaxies at redshifts greater than 0.5.Infrared JHK photometry for 81 3C radio galaxies is presented.These cover the redshift range from 0.03 to at least 1.6. By analysis of the colour-redshift and magnitude-redshift diagrams it is shown that the spectral energy distributions of the radio galaxies at redshifts of order unity are different from those of nearby galaxies. Evidence is presented which argues strongly in favour of the hypothesis that these differences are caused by changes in the stellar populations of the galaxies and the observations are re-examined in the light of models for the evolution of stellar populations. It is shown that both active and passive evolution (i.e. with and without star formation activity) is occurring in these galaxies.Infrared photometry for 39 radio sources selected to be 5-10 times fainter in the radio than the 3C sources is presented and discussed. Particular attention is paid to those radio sources whose identifications have not yet been seen in the optical waveband as these are probably very high redshift radio galaxies. Possible constraints on the epoch of galaxy formation are discussed.Surface photometry of ten nearby 3C radio galaxies is used to compare these galaxies with other samples of giant elliptical galaxies. Some statistical differences are found and these suggest that dynamical evolution has not been as important in the radio galaxies as in the others. This is probably related to their different local environments. Some consequences of this are discussed.