N ov 2 00 2 A single population scheme for FR I and FR II radio sources

The presence of two distant FR I radio galaxies in the Hubble Deep Field plus Flanking Fields indicates that the number density of these objects is about 10−50 times higher at z>1 than in the local universe. This is in strong contrast with the idea that FR Is undergo no cosmological evolution. We advocate that the cosmological evolution of radio sources may be independent of FR class, and instead solely a function of radio power. In this scenario the evolutionary properties of extragalactic radio sources can be fully described within a 'single population scheme'. 1 Historic background Early statistics of radio sources indicated that the space density of radio sources was much higher at early cosmological epochs than in the present universe (Ryle & Clarke 1961). Subsequent deeper radio surveys showed a turnover in the number counts, implying that this strong cosmological evolution must have been confined to only the most powerful radio sources (Lon-gair 1966), and a simple model was developed with two distinct population of radio sources; one low-luminosity non-evolving population, and one high-luminosity strongly-evolving population (Doroshkevich, Longair & Zeldovich 1970). Wall (1980) suggested that these two populations of non-evolving and strongly evolving radio sources correspond to the Fanaroff & Riley (1974) Class I and II galaxies respectively. This idea of a 'dual population scheme' has formed the basis of a comprehensive study by Jackson & Wall (1999), who in addition linked through orientation the non-evolving and strongly evolving populations of FR I and FR II radio galaxies to BL Lacs and flat spectrum quasars respectively. A similar scheme has recently been developed by Willott et al. (2001), but here a weakly evolving and a strongly evolving population