Resolved Spectroscopy of the Narrow‐Line Region in NGC 1068. III. Physical Conditions in the Emission‐Line Gas

Abstract

The physical conditions in the inner narrow-line region (NLR) of the Seyfert 2 galaxy NGC 1068 are examined using ultraviolet and optical spectra and photoionization models. The spectra were taken with the Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS), through the 01 ? 520 slit, covering the full STIS 1200-10000 ? waveband. The slit was centered on a position 014 north of the optical continuum peak (or hot spot) at a position angle of 202?, bisecting the brighter part of the biconical emission-line region. We have measured the emission-line fluxes for a region extending 38 northeast (~270 pc) to 18 southwest (~130 pc) of this point. The emission lines on each side show evidence of two principal kinematic components, one blueshifted with respect to the systemic velocity and the other redshifted (the kinematics were discussed in a separate paper). Based on the photoionization modeling results, we find that the physical conditions vary among these four quadrants. (1) The emission-line gas in the blueshifted northeast quadrant is photoionized by the hidden central source out to ~100 pc, at which point we find evidence of another source of ionizing radiation, which may be due to fast (~1000 km s-1) shocks resulting from the interaction of the emission-line knots and the interstellar medium. Interestingly, this occurs at approximately the location where the knots begin to show signs of deceleration. (2) The gas in the redshifted northeast quadrant is photoionized by continuum radiation that has been heavily absorbed by gas within ~30 pc of the central source. We find no strong evidence of the effects of shocks in this component. (3) The redshifted emission-line gas in the southwest quadrant is photoionized by unabsorbed continuum from the central source, similar to that in the inner ~100 pc of the blueshifted northeast quadrant. Finally, (4) the emission-line spectrum of the blueshifted southwest quadrant appears to be the superposition of highly ionized, tenuous component within the ionization cone and gas outside the cone, the latter photoionized by scattered continuum radiation. There are several implications of this complicated physical scenario. First, the hidden active nucleus is the dominant source of ionizing radiation in the inner NLR. The absorption of continuum radiation along the line of sight to the redshifted northeast quadrant may result from the intersection of the ionization cone and the plane of the host galaxy. Finally, the evidence for shock-induced continuum radiation at the point where the emission-line knots begin to decelerate indicates that the deceleration is due to the interaction of emission-line knots with slower moving gas, such as the interstellar medium of NGC 1068.