The Linear-size Evolution of Classical Double Radio Sources

Recent investigations of how the median size of extragalactic radio sources change with redshift have produced inconsistent results. Eales (1985) compared the radio and optical properties of a bright 3C and faint 6C sample and concluded that for a universe with 0 = 0 Dmed / (1 + z) 1:1 , with Dmed being the median size of the radio sources at a given epoch and z the redshift. Oort, Katgert, and Windhorst (1987b), on the other hand, from a comparison of the properties of a number of radio samples, found much stronger evolution, with Dmed / (1 + z) 3:3 0:5 for 0 = 0. In this paper we attempt to resolve the di erence. We have repeated the analysis of Eales using the much improved data, in particular, the virtually complete redshift information that now exists for the 6C sample. Con ning our analysis to FR2 sources, or \classical doubles", which we argue is the best-understood class of radio sources and the least likely to be a ected by selection e ects, we nd Dmed / (1 + z) 1:2 0:5 for 0 = 0 and Dmed / (1 + z) 1:7 0:4 for 0 = 1. Moreover, in contrast to earlier studies, we nd no intrinsic correlation between size and radio luminosity. We show that there is a selection e ect which a ects studies of this kind whose magnitude has not previously been realised, and has likely led to an overestimate in the strength of the size evolution found in previous investigations. Our complete redshift information allows us to gain insight into our result by plotting a radio luminosity-size (P -D) diagram for the 6C sample, the rst time this has been possible for a faint radio sample. The most obvious di erence between the 3C and 6C P -D diagrams (\the radio astronomers' H-R diagram") is the clump of sources in the 6C diagram at D 100 kpc, P151 10 10 W Hz 1 sr . These clump sources have similar sizes to the emission-line regions found around high-redshift radio galaxies, suggesting that the presence of dense line-emitting gas around high-redshift radio galaxies is responsible for the size evolution. We show that this explanation can quantitatively explain the observed size evolution, as long as there is either little X-ray emitting gas around these objects or, if there is, it is distributed in a similar way to the emission-line gas: highly anisotropic and inhomogeneous. Subject headings: galaxies: evolution, intergalactic medium | radio continuum: galaxies