Global Jet Watch

Abstract:

We discuss a unifying explanation for many 'trends with redshift' of radio galaxies which includes the relevance of their ages (time since their jet triggering event), and the marked dependence of their ages on redshift due to the selection effect of imposing a flux-limit. We briefly describe some important benefits which this 'youth-redshift degeneracy' brings.

Abstract:

We use three samples (3CRR, 6CE and 6C*) to investigate the radio luminosity function (RLF) for the 'most powerful' low-frequency selected radio sources. We find that the data are well fitted by a model with a constant ca-moving space density at high redshift as well as by one with a declining co-moving space density above some particular redshift. This behaviour is very similar to that inferred for steep-spectrum radio quasars by Willott et al (1998) in Line with the expectations of Unified Schemes. We conclude that there is as yet no evidence for a 'redshift cutoff' in the co-moving space densities of powerful classical double radio sources, and rule out a art-off at z less than or similar to 2.5.

Abstract:

Orientation-based unified schemes for radio-loud quasars and radio galaxies are discussed in the light of a new complete sample of identified radio sources - the 7C Redshift Survey. Selected at the low radio frequency of 151 MHz this sample is free of orientation biases and together with the 3CRR sample allows a direct comparison of the properties of radio galaxies and quasars. The fraction of quasars in complete samples is used to estimate the opening angle of the putative obscuring torus. Correlations between the extended radio luminosity and the optical continuum and narrow emission line luminosities are presented and discussed in terms of the physical processes occurring in radio-loud AGN.

Abstract:

We present a study of the trends in luminosity, linear size, spectral index, and redshift of classical double radio sources from three complete samples selected at successively fainter low radio-frequency flux-limits. We have been able to decouple the effects of the tight correlation between redshift and luminosity (inherent in any single flux-limited sample) which have hitherto hindered interpretation of the relationships between these four source properties. The major trends found are that (i) spectral indices increase with linear size, (ii) rest-frame spectral indices have a stronger dependence on luminosity than on redshift except at high (GHz) frequencies, and that (iii) the linear sizes are smaller at higher redshifts. We reproduce the observed dependences in a model for radio sources (born throughout cosmic time according to a radio-source birth function) whose lobes are fed with a synchrotron-emitting population (whose energy distribution is governed by compact hotspots), and which suffer inverse Compton, synchrotron and adiabatic expansion losses. In simulating the basic observed dependences, we find that there is no need to invoke any systematic change in the environments of these objects with redshift if the consequences of imposing a survey flux-limit on our simulated datasets are properly included in the model. We present evidence that for a radio survey there is an unavoidable `youth--redshift degeneracy', even though radio sources are short-lived relative to the age of the Universe; it is imperative to take this into account in studies which seemingly reveal correlations of source properties with redshift such as the `alignment effect'.