Circumnuclear Gas of a Quasar in Absorption and Emission Lines

We present a detailed analysis of the unusual absorption- and emission-line spectrum of the quasar SDSS J074850.39+442439.0 (hereafter J0748+4424) at a redshift of z = 2.18. The archival SDSS optical spectrum is abundant in narrow absorption lines (NALs) originating from mostly singly ionized iron, nickel, and silicon ions at both ground and excited levels. With the aid of the photoionization simulations, we find that these NALs can be reasonably well reproduced by a homogeneous gas slab with a neutral hydrogen column density of 1021.6-1021.9 cm-2 and a number density of 106.0-106.6 cm-3 illuminated by the quasar central engine with an ionization parameter of 10-2.8-10-2.1. We infer the gas is located at a distance of similar to 30-130 pc from the black hole. This circumnuclear gas can reveal itself in emission lines of a width of similar to 1000 km s-1. Such intermediate-width emission lines (IELs) are indeed dominant in the observed Ly alpha emission, as the Ly alpha broad emission line (BEL) is mostly eaten up by the expected damped Ly alpha absorption (DLA). The IELs, though rather weak, are also detected in H alpha and [O iii] emission lines. The IEL intensities largely agree with the predicted values by the simplified photoionization models using the parameters derived from the absorption lines and a covering fraction of 10%. The intrinsic DLA in J0748+4424, while itself contains abundant information on the circumnuclear environment, may serve as a "natural coronagraph" at the line of sight, exposing the IELs that are otherwise overwhelmed by the BELs.