The Physical Properties of Low-Redshift FeLoBAL Quasars: II. The Rest-Frame Optical Emission Line Properties

TL;DR

This study analyzes the optical emission lines of low-redshift FeLoBAL quasars, revealing a bimodal distribution in their properties and differences from unabsorbed quasars, challenging previous assumptions about their accretion rates.

Contribution

It is the first to show a bimodal distribution in the emission line properties of FeLoBAL quasars and compares them systematically with unabsorbed quasars.

Findings

FeLoBAL quasars exhibit a bimodal distribution in the E1 parameter.

FeLoBAL quasars have systematically broader Hbeta lines than unabsorbed quasars.

Discovery of low accretion-rate FeLoBAL quasars contradicts previous high-accretion assumptions.

Abstract

We report the results of analysis of the Hbeta emission-line region of a sample of thirty low-redshift (z<1) iron low-ionization broad absorption line quasars (FeLoBALQs). Eleven of these objects are newly classified as FeLoBALQs. A matched sample of 132 unabsorbed quasars was analyzed in parallel. The emission lines showed the well known anticorrelation between the [OIII] and FeII emission (Boroson & Green 1992). Using a summary statistic called E1 to quantify this anticorrelation, we found that while the distribution of E1 for the unabsorbed quasars has a single peak, the FeLoBALQs have a bimodal shape in this parameter. Previous studies have shown that the line emission properties of BAL and non-BAL quasars are consistent, and therefore the difference in the Hbeta region emission between FeLoBAL quasars and unabsorbed quasars is a new result. The two populations of FeLoBAL quasars are characterized by low and high bolometric luminosities and Eddington ratios. Some previous studies have suggested that BAL quasars are high accretion-rate objects, and therefore the discovery of the low accretion-rate branch of FeLoBAL quasars was unexpected. We also found that the Hbeta FWHM is systematically broader among the FeLoBALQs compared with the non-BAL quasars implying a higher inclination viewing angle or a dearth of low-velocity line-emitting gas.