IUE Observations of NGC 1068: The Extended High-Ionization Emission Region

UV emission morphology of the inner disk of NGC 1068 reveals extremely strong emission of the highly ionized species, N V λ1240, C IV λ 1550, and He II λ1640 in regions of distances up to 25"(2.5 kpc) from the nucleus. The strong emission from these species cannot be due to photoionization from OB stars and requires either photoionization from the nonthermal continuum of the AGN or yet some other ionization mechanism. The high-ionization emission region (HIER) is concentrated northeast of the nucleus but is completely absent in the southwest direction. Flux maps for N v, C IV, and He II covering the inner disk region of NGC 1068 are presented. The observations are consistent with the previously proposed dusty torus model of AGNs where the ionizing flux of the AGN is beamed in directions away from a dusty disk surrounding the central engine. The correlation of the strong HIER lines with starburst knots indicates that the cone of beamed radiation from the nucleus intersects the galactic plane. To explain the asymmetry of the HIER relative to the nucleus, we suggest that there is preferential clearing of the dense molecular gas, close to the nucleus, by the radio jet emanating from the nucleus toward the NE. Additional sweeping of the interstellar gas by the expanding CO ring and the rotating stellar bar provides further clearing of the gas in the inner disk at larger distance from the nucleus. This action allows EUV fluxes escaping from the central source to efficiently photoionize the gas in the northeast direction, especially in the [O III] Plume. The absence of these emission features in the southwest direction can be explained if there exists a large amount of absorbing material, SW of the nucleus, blocking the beamed UV radiation in this direction.