2021ApJ...914...13T

We present a detailed analysis of the broad absorption line system in the quasar SDSS J122017.06+454941.1, which are clearly detected in hydrogen Lyman series and metal lines, such as C IV, Si IV, Si III, Al III, and C II, with a similar velocity as that of the broad emission lines. We reliably measured the column densities of H I, Al III, and C II, and obtained a low limit to Si IV and Si III. With the help of the photoionization simulations, we found that the absorption gas has a hydrogen number density n_H ≃ 10^11.03 cm^–3 and a hydrogen column density N_H ≃ 10^21.0 cm^–2, and is exposed to the radiation with an ionization parameter U ≃ 10^–1.25, and thus located the absorber at ∼0.3 pc from the central supermassive black hole, remarkably similar to the radius of the broad-line region (BLR; 0.17-0.84 pc as estimated by the luminosity-radius relation) of the quasar. It is likely that our line of sight may happen to intercept the low-column part of the BLR with a high density similar to that of the inferred value of the absorber. We suggest that detection of Al III absorption line doublet in moderate quality quasar spectra could be a good indicator of dense gases, provided that the neutral hydrogen column density of the absorber is 15.4≲logN_HI(cm^–2)≲16.5.