Evolution of a Radio Source Population

Abstract

EDWARDS'S communication on the evolution of a radio source population misses the essential point of the discussion which has been given by Doroshkevich, Longair and Zeldovich1 and by Longair and Scheuer2. The counts of radio sources indicate that there were many more powerful radio sources per unit co-moving volume at earlier epochs than there are at the present time. We may express this by stating that the luminosity function of radio sources was different in the past in such a way that there were many more high luminosity sources. As discussed in section 5 of ref. 2, luminosity and density evolution are only two of an infinite number of possible ways of describing how these changes in the luminosity function have taken place with cosmological epoch. Evidently, knowing only the luminosity functions at different epochs the choice of luminosity or density evolution is impossible because, in the absence of any theoretical understanding of the nature of the evolution, the evolution function may in general be different for different luminosity classes as illustrated in Fig. 2 of ref. 2. Without further information about the sources themselves or without making additional assumptions, it cannot be concluded which, if either, of the forms of evolution is the better description of the evolution function. In the simple example which we gave to illustrate how the hypotheses of luminosity evolution and density evolution could lead to similar changes in the luminosity function with epoch, the highly restrictive assumption was made that the evolution function was the same for all luminosity classes and is applicable under the conditions discussed by Edwards. There is no reason why this assumption should bear any resemblance to the actual evolution of the radio source population and indeed the calculations of Doroshkevich, Longair and Zeldovich1 and of Ringenberg and McVittie3 indicate that the counts of radio sources can only be explained on the basis of differential evolution.