2007ApJ...663..781M

We present results of the near-infrared (IR) spectroscopy of six quasars whose redshifts range from 0.158 to 1.084. Combined with the satellite ultraviolet data, the relative line strengths of O I λ1304, O I λ8446, O I λ11287, and the near-IR Ca II triplet are given. In addition, the corresponding O I line strengths measured in normal Seyfert 1s and narrow-line Seyfert 1s are collected from the literature. These lines are thought to emerge from the same gas as do the Fe II lines, so they are good tracers of the Fe II emission region within a broad emission line region (BELR) in active galactic nuclei (AGNs). In order to reveal the physical condition within the relevant emission region, we performed photoionized model calculations and compared them to the observations. It suggests that a rather dense gas with density n_H∼10^11.5/cm3 is present at an outer portion of the BELR, illuminated by the ionizing radiation corresponding to an ionization parameter U∼10^–2.5 and is primarily responsible for the observed O I, Ca II, and Fe II lines, based on the resemblance of their profiles. The three O I lines are proven to be formed through Lyβ fluorescence and collisional excitation. We also show that the λ1304 bump typically observed in AGN spectra consists of the comparable contributions of O I and Si II multiplets, and we discuss the origin of such a strong Si II emission. The results are interpreted in the context of the locally optimally emitting cloud (LOC) scenario to find the plausible gas distribution within the BELR as a function of distance from the central source and density.