The circumnuclear environment of nearby, noninteracting Seyfert galaxies

An investigation into the physical conditions prevailing in the regions immediately surrounding the active nuclei in 20 nearby, noninteracting Seyfert galaxies is reported. CCD interference-band images isolating the bright emission lines of Hα + N II and O III λ5007 have been obtained to search for spatially extended circumnuclear emission regions. Long-slit, low-resolution spectrophotometry of interesting cases was used to probe the ionization state of the extended emission regions. Only three out of nine Seyfert 1 galaxies have spatially extended ionized gas regions, compared with eight out of 11 Seyfert 2 galaxies. Enhanced circumnuclear star formation is uncommon to both Seyfert 1 and 2 galaxies. Extended emission in Seyfert 1 galaxies has nearly the same morphology in both Hα + N II and O III emission. In the Seyfert 2 galaxies, the Hα + N II and O III images show different extended emission morphologies. The O III emission regions appear as either one- or two-sided structures, four of which are resolved into distinct cones of high-ionization gas emanating from the active nucleus. Spectra of these extended regions show the gas to be in a high excitation state, consistent with photoionization by a hard-photon spectrum, possibly the nuclear continuum. For Seyfert 1 and 2 galaxies with published radio maps, the radio continuum and optical emission-line structures appear roughly aligned, but are not correlated knot-for-knot. The morphology and ionization of these regions suggests anisotropy of the nuclear ionizing continuum. Ionizing radiation cones like those observed are predicted by models of Seyfert nuclei in which Seyfert 2 galaxies are Seyfert 1 nuclei "hidden" from our line of sight. However, this model may only apply to some Seyfert 2 galaxies, since there are significant differences between Seyfert 1 and 2 galaxies in their circumnuclear properties which are not easily reconciled with this simple "unified" picture.