Dependence of Linear Sizes and Spectral Indices of Extended Radio Galaxies on Redshift and Radio Luminosity

Abstract

Apart from the well known evolution in the space density of powerful radio sources, the maximum linear sizes and the spectral indices of extended radio sources could also evolve with epoch and it is important to investigate their epoch-dependence in order to understand the cosmological evolution of radio sources. Evidence for size evolution (in the sense that sources were smaller at earlier epochs) has been presented both from the angular size – redshift relation (θ – z) for quasars and from the angular size – flux density relation (θ – S) for all radio sources. Doubts have sometimes been expressed, however, mainly because it has been difficult to investigate the epoch dependence independent of a possible dependence on radio luminosity. Reasonably reliable estimates of the distances of radio sources have generally been limited to a relatively small range in flux density (S), so that high redshift sources are also of high luminosity. An inverse correlation between linear size (l) and radio luminosity (P) is therefore hard to distinguish from a genuine evolution with epoch. A similar situation appears to hold for the spectral indices (α) of extended radio sources, which have generally been assumed to get steeper at higher radio luminosities (eg. Laing and Peacock 1980) but a dependence on epoch cannot at present be ruled out (Katgert-Merkelijn et al. 1980).