2002MNRAS.331L..13R

We present accurate measurements of the physical conditions in five powerful radio galaxies, as derived from deep, long-slit spectroscopic observations. All five objects show prominent extended line emission, and have X-ray luminosities similar to those of isolated elliptical galaxies. The data are high enough quality that the electron density and temperature can be measured at several positions across the emission-line nebulae.

We subtract a model continuum comprising a combination of a 15-Gyr stellar template, a young stellar template and a power law, so as to be better able to measure faint diagnostic lines. Electron temperatures measured from the [Oiii](4959+5007)/4363 line ratio are in the range 10000<T_e <20000K, whilst [Sii](6716/6731) densities fall between 100-500cm^–3. Using these values, we find pressures within the line-emitting clouds a factor of 10-100 times higher than expected for pressure balance with the hot X-ray haloes of the host galaxies.

Previous studies of sources that show significant evidence of jet-cloud interactions, both in terms of their kinematics and ionization, have concluded that the overpressure is a result of the warm, line-emitting gas being compressed by the radio cocoon; however, there is no evidence that the radio jet is influencing the emission-line regions in four of our five objects.

We suggest that it is plausible that the line-emitting clouds have not yet relaxed into pressure equilibrium from their initial photoionization by the central active galactic nucleus.