Physical conditions in the inner narrow-line region of the Seyfert 2 galaxy NGC 1068

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 are Hubble Space Telescope archive data obtained with the Faint Object Spectrograph (FOS). We selected spectra of four regions, taken through the 0." 3 FOS aperture covering the full FOS 1200-6800 Angstrom wave band. Each region is approximately 20 pc in extent, and all are within 100 pc of the apparent nucleus of NGC 1068. The spectra show similar emission-line ratios from a wide range of ionization states for the most abundant elements. After extensive photoionization modeling, we interpret this result as an indication that each region includes a range of gas densities, which we included in the models as separate components. Supersolar abundances were required for several elements to fit the observed emission-line ratios. Dust was included in the models, but apparently dust-to-gas fraction varies within these regions. The low-ionization lines in these spectra can be best explained as arising in gas that is partially shielded from the ionizing continuum. Although the predicted line ratios from the photoionization models provide a good fit to the observed ratios, it is apparent that the model predictions of electron temperatures in the ionized gas are too low. We interpret this as an indication of additional collisional heating due to shocks and/or energetic particles associated with the radio jet that traverses the NLR of NGC 1068. The density structure within each region may also be the result of compression by the jet.