An Emission-Line Imaging Survey of Early-Type Seyfert Galaxies. II. Implications for Unified Schemes

Abstract

We report simulations of emission-line morphologies and results of a large imaging survey of a complete sample of Seyfert galaxies in early- type hosts in the emission lines of [O III] λ5007 and Hα + [N III] λλ6548, 6583 and the nearby continua. These simulations and observations are compared in order to test models in which the apparent Seyfert class depends on the orientation of the active nucleus with respect to the observer. In unified models, the active nucleus is surrounded by a thick, dusty gaseous torus. The nuclear ionizing radiation is then expected to escape in oppositely directed cones along the axis of the torus. We have, therefore, performed simulations in which ambient gas is ionized by a bi-conical radiation field. When the ambient gas is distributed in a sphere or spheroid, V- shaped projections are expected whenever our line of sight is outside the cones of ionizing radiation. For line of sights within the cones, "halo"-like morphologies are produced. For such gas distributions, the measured opening angle of the V-shaped morphology is equal to or larger than the true opening angle of the radiation field. On the other hand, when the gas is distributed in a thin disk, V-shaped projections are produced the majority of the time, even when our line of sight is within the photon cone. The measured opening angle in the case is usually smaller than the true opening angle of the photon cone. For both spherical and thin disk gas distributions, the projected linear extent of the ionized gas tends to be smaller for lines of sight within the cones. From the emission-line images, we find that extended emission-line gas is very common in Seyfert galaxies, with ~80% of the galaxies showing extension in our [O III] λ5007 images and ~100% showing extension in Hα + [N II]. The percentage of galaxies extended in these lines is comparable for Seyfert 1s and 2s. There is a strong correlation between the line fluxes of the unresolved core and the extended emission, suggesting the extended emission is ionized by the same source that ionizes the nuclear gas. There is also a correlation between emission- line extent and luminosity. The total [O III] λ5007 luminosities of the Seyfert 1s in our sample are similar to those of the Seyfert 2s, while the total Hα + [N II] emission is larger in Seyfert 1s than Seyfert 2s. This result is not surprising since the broad component to the Hα line contributes a significant amount of flux in the Seyfert 1s. In order to compare the extended emission-line luminosities of the two Seyfert types, we have subtracted an unresolved nuclear component from the images. We find a trend for the extended emission in both [O III] λ5007 and Hα + [N II] to be more luminous in the Seyfert 1s than the Seyfert 2s. However, this trend is of only marginal significance in the complete sample. In general, it is difficult, if not impossible, to distinguish a Seyfert 1 galaxy from a Seyfert 2 galaxy based only on its emission-line characteristics. While linear, bipolar, or V-shaped morphologies are seen in some objects, for one-third of the sample the emission-line structures are "halo "-like. These "halo" morphologies are not expected for Seyfert 2s in the unified model. Many of the "halo" Seyfert 2s do display linear or V-shaped structures in their excitation maps (i.e., distributions of [O III]/Hα + [N II]), so they may still be consistent with orientation - dependent models. However, the emission-line morphologies of at least a few Seyfert 1s appear inconsistent with those expected in the simplest unified schemes. In our complete sample, the extent of the [O III] λ5007 emission at a given surface brightness is larger in the Seyfert 2s than the Seyfert 1s. However, this difference disappears if the two most extended Seyfert 2s are omitted. We also find evidence for structure in many of the continuum color maps. In ~40% of the galaxies with color maps, red (V - R > 1.0) features are found. The majority of these red regions are unresolved structures located at the nucleus. These features are found in galaxies of all inclinations, but occur almost exclusively in type 2 Seyfert galaxies. These characteristics are consistent with reddening by ~100 pc scale, dusty tori. A less likely alternative is that these structures represent a very red stellar population (later than K5). Diffuse blue (V - R < 0.5) features are seen in some Seyfert 2 galaxies. These blue excesses tend to be spatially coincident with the high-excitation gas, suggesting an origin related to the nuclear activity. The blue excesses could originate from a number of different processes, including scattered nuclear light or an extended nonstellar continuum associated with the ionized gas (e.g., shock waves).