Exploring the Inner Narrow-Line Regions of Seyfert Galaxies

In HST/STIS optical spectra of Seyfert galaxies, there is often a bright knot of [OIII] emission in the inner NLR. In the case of the Seyfert 1 galaxy NGC 4151, we have shown that the emission-line gas may be associated with the the mass outflow detected in absorption in UV spectra, which suggests that we are probing regions close to the AGN. Here, we present results for the luminous Seyfert 2 galaxy, Mrk 573. The spectra reveal the presence of lines from a wide range of ionization states, which is indicative of the heterogenous nature of the emission-line gas. As in our studies of other Seyfert galaxies, including NGC 1068, NGC 4151, and Mrk 3, there is evidence for emission from low-ionization gas outside the nominal emission-line bicone. Finally, based on photoionization modeling, we find that the central emission-line knot in Mrk 573 is tens of parsecs from the AGN, similar to other Seyfert 2 galaxies, which suggests that the material that is obscuring the central AGN in Seyfert 2s must have a similar radial extent. This presents a new challenge for the unified model of Seyfert galaxies, which posits that the obscuration occurs on parsec scales.