Chandra Observations of Candidate “True” Seyfert 2 Nuclei

Abstract

The unification model for AGNs posits that Seyfert 2 galaxies are intrinsically like Seyfert 1 galaxies, but that their broad-line regions (BLRs) are hidden from our view. A Seyfert 2 nucleus that truly lacked a BLR, instead of simply having it hidden, would be a so-called true Seyfert 2. No object has as yet been conclusively proven to be one. We present a detailed analysis of four of the best true Seyfert 2 candidates discovered to date: IC 3639, NGC 3982, NGC 5283, and NGC 5427. None of the four have a broad Hα emission line, in either direct or polarized light. All four have rich, high-excitation spectra, blue continua, and HST images showing them to be unresolved sources with no host-galaxy obscuration. To check for possible obscuration on scales smaller than that resolvable by HST, we obtained X-ray observations using Chandra. All four objects show evidence of obscuration and therefore could have hidden BLRs. The picture that emerges is of moderate to high, but not necessarily Compton-thick, obscuration of the nucleus, with extranuclear soft emission extended on the hundreds of parsec scale that may originate in the narrow-line region. Since the extended soft emission compensates, in part, for the nuclear soft emission lost to absorption, both absorption and luminosity are likely to be severely underestimated unless the X-ray spectrum is of sufficient quality to distinguish the two components. This is of special concern where the source is too faint to produce a large number of counts, or where the source is too far away to resolve the extended soft X-ray-emitting region. © 2007. The American Astronomical Society. All rights reserved.