A new sample of giant radio galaxies from the WENSS survey II. A multi-frequency radio study of a complete sample:Properties of the radio lobes and their environment

We have formed a complete sample of 26 low redshift (z less than or similar to 0.3) giant radio galaxies (GRGs) from the WENSS survey, selected at flux densities above 1 Jy at 325 MHz. We present 10.5-GHz observations with the 100-m Effelsberg telescope of 18 sources in this sample. These observations, together with similar data of the remaining eight sources, are combined with data from the WENSS, NVSS and GB6 surveys to study the radio properties of the lobes of these sources at arcminute resolution. We investigate radio source asymmetries, equipartition energy densities in the lobes, the presence of lobe pressure evolution with redshift, the spectral age and the density of the environments of these sources. We iind that the arm length asymmetries of GRGs are slightly larger than those of smaller sized 3CR radio galaxies and that these are difficult to explain as arising from orientation effects only. We also find indications that the lobes of the GRGs, despite their large sizes, are still overpressured with respect to their environment. Further, we argue that any evolution of lobe pressure with redshift in these large sources (e.g. Cotter 1998) is due to selection effects. For sources which could be used in a spectral ageing analysis, we find spectral ages which are large, typically a few times 10(7) Sr. This is comparable to earlier studies of some giant sources and indicates that such large spectral ages are common for this class of radio source. The advance velocities of the radio lobes are typically a few percent of the speed of light; which is higher than those found for smaller, low power (<10(26.5) W Hz(-1) at 178 MHz) radio sources, and more comparable to higher power radio sources. This suggests that the GRGs in our sample are the oldest members of the group of relatively high power radio sources whose radio powers have evolved to their currently observed lower values (cf. Kaiser et al. 1997).