Multifrequency models for the cosmological evolution of extragalactic radio sources

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Abstract
This paper presents new models for the epoch dependence of the luminosity function of extragalactic radio sources. These models have been derived on the assumption that radio sources may be divided into two distinct populations: the extended steep-spectrum sources and the compact flat-spectrum sources. By using the correlation between luminosity and spectral index for the steep-spectrum sources, the luminosity functions defined at different frequencies may be related, allowing a single model to be tested against all available source counts, luminosity distributions and identification data. To obtain models that are consistent with the data, the luminosity functions for each of the two spectral classes are expressed as free series expansions in luminosity and redshift, and the expansion coefficients that yield the best-fitting models are found iteratively. Different expansion forms are considered in order to estimate the uncertainty in the luminosity function; it is found that, in most areas of the P–z plane, the luminosity function is poorly defined and a variety of evolutionary histories are possible. Within the regions which are well defined by the redshift information from bright-source samples, however, both the steep-spectrum and flat-spectrum sources appear to behave similarly: powerful sources of either type undergo strong evolution.

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