Low-Ionization Emission Regions in Quasars: Gas Properties Probed with Broad O I and Ca II Lines

TL;DR

This study investigates the properties of low-ionization gas in quasars by analyzing specific emission lines, revealing consistent gas densities and diverse ionization parameters across different quasar environments.

Contribution

It provides new insights into the physical conditions of the line-emitting gas in quasars using a comprehensive analysis of O I and Ca II emission lines across a wide range of redshifts and luminosities.

Findings

Gas density is similar across quasars, indicated by Ca II and O I flux ratios.

O I line widths are consistent over a broad luminosity range, suggesting a kinematic emission region.

Diversity in ionization parameter inferred from O I line ratio scatter.

Abstract

We have compiled the emission-line fluxes of O I 8446, O I 11287, and the near-IR Ca II triplet (8579) observed in 11 quasars. These lines are considered to emerge from the same gas as do the Fe II lines in the low-ionized portion of the broad emission line region (BELR). The compiled quasars are distributed over wide ranges of redshift (0.06 < z < 1.08) and of luminosity (-29.8 < M_B < -22.1), thus representing a useful sample to investigate the line-emitting gas properties in various quasar environments. The measured line strengths and velocities, as functions of the quasar properties, are analyzed using photoionization model calculations. We found that the flux ratio between Ca II and O I 8446 is hardly dependent on the redshift or luminosity, indicating similar gas density in the emission region from quasar to quasar. On the other hand, a scatter of the O I 11287/8446 ratios appears to imply the diversity of the ionization parameter. These facts invoke a picture of the line-emitting gases in quasars that have similar densities and are located at regions exposed to various ionizing radiation fluxes. The observed O I line widths are found to be remarkably similar over more than 3 orders of magnitude in luminosity, which indicates a kinematically determined location of the emission region and is in clear contrast to the well-studied case of H I lines. We also argue about the dust presence in the emission region since the region is suggested to be located near the dust sublimation point at the outer edge of the BELR.