Origin of Ultraviolet Intermediate-width Emission Lines in Partially Obscured Quasar SDSS J151653.22+190048.2

Abstract

The quasar SDSS J151653.22+190048.2 (J1516+1900) exhibits very peculiar spectral characters in the observed ultraviolet, optical, and infrared bands: the full widths at half maximum (FWHM) of its optical Hα, Hβ, and infrared Paα and Paβ emission lines are all larger than 5000km/s, and the equivalent widths of these emission lines also approach to those of normal quasars; while the ultraviolet emission lines (Lyβ, OVI, Lyα, NV, SiIV, and CIV) are dominated by intermediate-width emission lines (IELs) with a FWHM of ∼ 1700km/s. This phenomenon can be explained as the effect of partial obscuration: the broad emission lines (BELs) in the ultraviolet waveband are heavily suppressed by the dust extinction, and thus the IELs become prominent; while the dust extinction is not significant in optical and infrared bands, and the IELs can hardly be detected due to the brightness of BELs. Using the IEL width and central supermassive black hole mass MBH∼5.75×108 M⊙ of J1516+1900, and assuming that the intermediate-width emission line region (IELR) is virialized, we estimate the distance of the IEL region to the central black hole is about 1.6pc. On the other side, by combining the photoionization model calculations and observed IELs, we find that the IELR has a density of ∼1012 cm−3, and the ionization parameter of ∼10−0.65, and that its distance to the central region is ∼0.016pc, which is only one percent of the virialized distance. This contradiction can provide an important clue for studying the geometry, physical condition, and origin of the IELRs in active galactic nuclei.