Classical double radio galaxies and their gaseous environments

A sample of classical double radio galaxies has been constructed to study the effect of environment on radio sources. The sample consists of radio galaxies in cluster and noncluster environments and includes galaxies and clusters at both high (z ∼ 0.5) and low (z ∼ 0) redshift. Most of these radio galaxies are intermediate-power FR II sources with 408 MHz powers in the range from 10³² to 10³⁵ h^-2 ergs s^-1 Hz^-1, where the upper bound corresponds roughly to a 178 MHz power of about 10²⁷ h^-2 W Hz^-1 sr^-1. Comparisons are made between the properties of FR II radio galaxies in cluster and noncluster environments, and between X-ray clusters with and without FR II radio galaxies. The principal results are the following: 1. Most low-redshift FR II galaxies in clusters appear to be similar to FR II galaxies in group or field environments in terms of radio power, optical properties of the host galaxy, and nonthermal pressure of the radio bridge. 2. High-redshift FR II galaxies are all quite similar and do not appear to vary with galactic environment. The radio powers and emission-line luminosities of the host galaxies of high-redshift FR II sources are higher on average than their low-redshift counterparts, but there is also overlap, and the nonthermal pressures of the radio bridges appear to be independent of redshift. 3. The nonthermal pressures of the bridges of FR II sources appear to be similar to the thermal pressures of the ICM around them. This result, if confirmed by a larger sample, would allow the bridges of FR II sources to be used as probes of their gaseous environments. 4. The fact that most FR II galaxies have similar nonthermal pressures, irrespective of galactic environment and redshift, indicates that the gaseous environments around them are also similar. Typical nonthermal pressures of these FR II sources are much lower than typical thermal pressures of the ICM in X-ray bright clusters, suggesting that FR II sources are generally in environments with gas pressures much less than typical low-redshift ICM pressures. 5. X-ray data at low redshift show that clusters with FR II sources tend to be underluminous in the X-ray compared to clusters without FR II sources. This suggests that the ICM pressures in these clusters are relatively low, consistent with the results obtained from the analysis of the nonthermal pressures of FR II sources. 6. Unfortunately, the X-ray data for high-redshift clusters with FR II sources are inconclusive because of the large number of upper bounds involved and possible AGN contribution to the X-ray luminosity. However, the fact that most high-redshift FR II radio galaxies have nonthermal pressures similar to their low-redshift counterparts indicates that the gaseous environments around them are similar, in which case most high-redshift clusters with FR II sources should be underluminous X-ray emitters, as are their low-redshift counterparts. 7. Thus, the evolution in the clustering strength around FR II sources toward high-redshift is likely to be closely linked to an evolution of the state of the intracluster medium. Namely, there are more clusters of low gas pressure at high redshift, and thus more FR II sources can appear in these clusters. This is consistent with the negative evolution of the cluster X-ray luminosity function with redshift, and the fact that many high-redshift clusters have much lower X-ray luminosities than optically similar low-redshift clusters.