Strong optical and UV intermediate-width emission lines in the quasar SDSS J232444.80–094600.3: dust-free and intermediate-density gas at the skin of dusty torus?

Emission lines from the broad emission line region (BELR) and the narrow emission line region (NELR) of active galactic nuclei (AGNs) have been extensively studied. However, emission lines are rarely detected between these two regions. We present a detailed analysis of quasar SDSS J232444.80–094600.3 (SDSS J2324–0946), which is remarkable for its strong intermediate-width emission lines (IELs) with FWHM ≈ 1800 km s⁻¹. The IEL component is present in different emission lines, including the permitted lines Lyα λ1216, CIV λ1549, semiforbidden line [CIII] λ1909, and forbidden lines [OIII] λλ4959, 5007. With the aid of photo-ionization models, we found that the IELs are produced by gas with a hydrogen density of n_H ∼ 10^6.2 ∼ 10^6.3 cm⁻³, a distance from the central ionizing source of R ∼ 35 – 50 pc, a covering factor of ∼ 6%, and a dust-to-gas ratio of ≤ 4% that of the SMC. We suggest that the strong IELs of this quasar are produced by nearly dust-free and intermediate-density gas located at the skin of the dusty torus. Such strong IELs, which serve as a useful diagnostic, can provide an avenue to study the properties of gas between the BELR and the NELR.