A SAMPLE OF 6C SOURCES SELECTED AT THE PEAK OF THE SOURCE COUNTS .3. AN INVESTIGATION OF THE VARIATION OF THE SIZES OF RADIO-SOURCES WITH COSMIC EPOCH

The radio (Paper I) and optical data (Paper II) for a sample of 67 6C radio sources with flux density ≈2 Jy at 151 MHz are compared with data for a sample of bright sources. The 6C sources are found to be at higher redshifts but have similar radio luminosities, except that there are fewer 6C sources with very low luminosities. There is little evidence in samples with this level of flux density of the strong correlation between spectral index and radio luminosity or redshift present in brighter samples. An investigation has been made into whether the number density evolution of radio galaxies is different from that of quasars of the same radio luminosity. This was inconclusive; more spectroscopic redshifts will allow a much more stringent test of this possibility. A new method is presented for investigating possible cosmological evolution in the physical sizes of sources, and it is shown that such evolution is present, in the sense that the median size D of sources was smaller at earlier epochs, both for Ω0=0 and for Ω0=1. Because of the parameters chosen for the faint sample this result should not be due to selection effects. If a function D ?(1+z)−n is fitted to the data, n = 1.45 ± 0.4 for Ω0=1, and n = 1.1 ± 0.5 for Ω0=0. No evidence is found for a correlation between luminosity and physical size which is not merely due to both being correlated with redshift. It does not seem that this evolution can be explained either by the increase with redshift of the energy density in the microwave background or by invoking special evolutionary tracks through the P-D diagram. © 1985 Royal Astronomical Society. Provided by the NASA Astrophysics Data System.