Polarized Narrow-Line Emission from the Nucleus of NGC 4258

Abstract

The detection of polarized continuum and line emission from the nucleus of NGC 4258 by Wilkes et al. in 1995 provides an intriguing application of the unified model of Seyfert nuclei to a galaxy in which there is known to be an edge-on, rotating disk of molecular gas surrounding the nucleus. Unlike most Seyfert nuclei, however, NGC 4258 has strongly polarized narrow emission lines. To further investigate the origin of the polarized emission, we have obtained spectropolarimetric observations of the NGC 4258 nucleus at the Keck II telescope. The narrow-line polarizations range from 1.0% for [S II] λ6716 to 13.9% for the [O II] λλ7319, 7331 blend, and the position angle of polarization is oriented nearly parallel to the projected plane of the masing disk. A correlation between critical density and degree of polarization is detected for the forbidden lines, indicating that the polarized emission arises from relatively dense (ne 104 cm-3), radially stratified gas. An archival Hubble Space Telescope narrowband [O III] image shows that the narrow-line region has a compact, nearly unresolved core, implying a FWHM size of 2.5 pc. We discuss the possibility that the polarized emission might arise from the accretion disk itself and become polarized by scattering within the disk atmosphere. A more likely scenario is an obscuring torus or strongly warped disk surrounding the inner portion of a narrow-line region that is strongly stratified in density. The compact size of the narrow-line region implies that the obscuring structure must be smaller than about 2.5 pc in diameter.