8 Conclusions

We have studied the radio properties of a complete sample of 26 giant radio galaxies. These have been selected from the WENSS survey at a flux density above 1 Jy at 325 MHz and an angular size above $5\hbox{$^\prime$ }$. We have presented new radio observations of 18 of these sources at a frequency of 10.5 GHz, obtained with the 100-m Effelsberg telescope. These have been used in combination with available data from the WENSS, NVSS and GB6-surveys for a low-resolution (arcmin-scale) multi-frequency radio investigation of the properties of these sources. We have found the following:

1.

The armlength asymmetries of GRGs are different from those of 3CR radio galaxies. The GRGs tend to be more asymmetric, which cannot be explained as an orientation effect only. We find that in 15 out of the 20 FRII-type GRGs in our sample, the radio lobe which has the steepest spectrum between 325 MHz and 10.5 GHz also has the shortest arm. We conclude that this cannot be result of a difference in the expansion-rate of the two radio lobes, thereby excluding asymmetries in the environment as the major cause of this effect;

2.

In profiles of the spectral index as a function of distance from the hotspots in the lobes, we find significant steepening of the spectrum away from the hotspot only in a few cases. Fitting these profiles with model spectra yields a typical advance velocities of $\sim\!0.04c$, and a spectral age of 80 Myr. Such large ages agree with what has been found for several other GRGs in the past;

3.

We find a dichotomy between powerful ( P₁₇₈ > 10^26.5 WHz^-1ster^-1) and less powerful sources when we compare their lobe advance velocities, as deduced from spectral ageing studies, with their linear size. First, we find that for linear sizes around 100 kpc, the high power sources typically have much higher lobe velocities than the low power sources. This dichotomy disappears for larger sources. Further, less powerful sources show a strong correlation between source size and lobe advance velocity, which extends all the way to the largest sources, the GRGs. This may be largely due to the following: Slowly advancing sources may never grow out to Mpc sizes within the lifetime of the radio active phase of the AGN;

4.

Using the measured advance velocities, ages and energy contents of the lobes of the GRGs, we find a typical particle density in front of the lobes of a few times 10^-5 cm^-3. This is in agreement with earlier results on the density around the lobes of GRGs using similar methods (Mack et al. 1998). Assuming a temperature of a few times 10⁶ K, the thermal pressure in a medium with a particle density of $4\ 10^{-5}$ would be $\sim\!2\ 10^{-14}$ dyn cm^-2. This lies below the typical lobe pressures we find from equipartition arguments. Profiles of the equipartition energy density along the radio axis in the lobes indicate that there often is a strong pressure gradient in the lobes, with the hotspots having the highest pressures. Also this indicates that the radio lobes are overpressured with respect to their environment;

5.

The lobe pressures show a strong correlation with redshift. This has been noted before by, e.g., Subrahmanyan & Saripalli (1993) and Cotter (1998). We show that the correlation in our sample can be explained by two effects: The use of a flux density and linear size limited sample and the method by which the equipartition lobe pressures are calculated. We find therefore no evidence for a cosmological evolution of the IGM pressure between z=0 and z=0.3. Our observations agree with a present day value of the IGM pressure of $\sim 10^{-14}$ dyn cm^-2.

This has been the first study of its kind employing a complete and relatively large sample of GRGs. The main result is that on basis of the data presented here we find that GRGs are both old sources, in terms of their spectral age, and that they are situated in a relatively low density environment, but also that neither of these two properties are extreme. Therefore, based on the study presented here, their large size probably results from a combination of these properties.

Acknowledgements

KHM was supported by the Deutsche Forschungsgemeinschaft, grant KL533/4-2 and by the European Commission, TMR Programme, Research Network Contract ERBFMRXCT96-0034 "CERES''. The Westerbork Synthesis Radio Telescope (WSRT) is operated by the Netherlands Foundation for Research in Astronomy (NFRA) with financial support of the Netherlands Organization for Scientific Research (NWO). The National Radio Astronomy Observatory (NRAO) is operated by Associated Universities, Inc., and is a facility of the National Science Foundation (NSF). This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. We acknowledge the use of NASA's SkyView facility (http://skyview.gsfc.nasa.gov) located at NASA Goddard Space Flight Center.