Detection of the Intermediate-width Emission Line Region in Quasar OI 287 with the Broad Emission Line Region Obscured by the Dusty Torus

TL;DR

This study provides evidence for the intermediate-width emission line region (IELR) in a partially obscured quasar, suggesting it originates from the inner surface of the dusty torus and can be studied via its unique emission features.

Contribution

It offers the first convincing detection and analysis of the IELR in a quasar, linking it to the dusty torus and proposing a natural 'coronagraph' effect for studying such regions.

Findings

IELR detected in quasar OI 287 with prominent ultraviolet lines

Estimated IELR distance of ~2.9 pc from the black hole

IELR physical conditions consistent with dusty torus environment

Abstract

The existence of intermediate-width emission line regions (IELRs) in active galactic nuclei has been discussed for over two decades. A consensus, however, is yet to be arrived at due to the lack of convincing evidence for their detection. We present a detailed analysis of the broadband spectrophotometry of the partially obscured quasar OI 287. The ultraviolet intermediate-width emission lines (IELs) are very prominent, in high contrast to the corresponding broad emission lines (BELs) which are heavily suppressed by dust reddening. Assuming that the IELR is virialized, we estimated its distance to the central black hole of $\sim 2.9$ pc, similar to the dust sublimation radius of $\sim 1.3$ pc. Photo-ionization calculations suggest that the IELR has a hydrogen density of $\sim 10^{8.8}-10^{9.4} ~ \rm cm^{-3}$, within the range of values quoted for the dusty torus near the sublimation radius. Both its inferred location and physical conditions suggest that the IELR originates from the inner surface of the dusty torus. In the spectrum of this quasar, we identified only one narrow absorption-line system associated with the dusty material. With the aid of photo-ionization model calculations, we found that the obscuring material might originate from an outer region of the dusty torus. We speculate that the dusty torus, which is exposed to the central ionizing source, may produce IELs through photo-ionization processes, while also obscure BELs as a natural "coronagraph". Such a "coronagraph" could be found in a large number of partially obscured quasars and be a useful tool to study IELRs.