Extended Emission-Line Gas in Distant 3CR Radio Galaxies

We report on the results of long-slit spectroscopy and interference filter imaging of powerful 3CR radio galaxies. Narrow-band [OII]A3727 imaging often reveals extensive regions of extranuclear emission associated with these objects. The extended [OII] emission is distributed on scales of several tens of kpc, it is often more luminous than the nuclear emission, and shows large velocity gradients. We discuss the physical conditions in these nebulae and their relation to the radio source structure. In particular, we examine the ionization mechanism for the gas, and find that photoionization models reproduce the observed emission line strengths reasonably well. The source of the ionizing photons, however, is not clear; the large luminosities of the extended gas suggesting that some form of in situ ionization may be occurring. Compared to the low-z cooling flow filaments, the 3CR emission regions are larger, more luminous, and generally show higher excitation. Futhermore, the distant radio galaxies show evidence for much stronger interactions with their radio sources than is seen in nearby cooling flow filaments.