The ionization structure of the narrow-line region of NGC 1068 and its relationship to the extended radio emission

Hubble Space Telescope (HST)/WFPC2 imaging has been used to investigate the emission-line structure of the narrow-line region (NLR) of the Seyfert 2 galaxy NGC 10618 and its relationship to the extended radio emission. We find that the morphology and the ionization properties of the NLR of NGC 1068 are dominated by the interaction with the radio jet, in full agreement with similar results obtained for other Seyfert galaxies with extended radio emission. The emission lines originate in two distinct regions: a large-scale cocoon enveloping the northeastern radio lobe and a small-scale broken network of filaments and knots surrounding the radio jet. The material along the radio jet, although denser than the surrounding gas, is in a much higher ionization state than the remainder of the NLR. We believe that this requires the presence of a source of ionizing radiation that locally dominates over the nuclear emission, e.g., free-free emission from the gas heated by the interaction with the radio jet. At a distance of approximately 4 '' (similar to 300 pc) from the nucleus, corresponding to the transition in the radio structure from jetlike to lobelike, the ionization structure of the NLR shows a sharp and well-defined boundary between an inner low-ionization zone and an outer higher ionization zone. This ionization change is easily explained if there is a density increase where the jet enters the lobe, presumably due to the compression of the backflowing radio cocoon at the jet working surface. A color image reveals the presence of a nuclear dust lane projected onto the location of the hidden nucleus. Nuclear dust lanes appear to be ubiquitous in Seyfert 2 galaxies observed with FIST and can be naturally associated with the material responsible for the obscuration of the active nucleus.