The environment of weak emission-line quasars

TL;DR

This study investigates the nature of weak emission-line quasars (WLQs) by comparing their Baldwin effect and emission-line properties with normal quasars, revealing differences in broad line region covering factors and challenging the super-Eddington hypothesis.

Contribution

It provides new insights into the BLR structure of WLQs, showing their low covering factor and distinct Baldwin effect relationship compared to normal quasars.

Findings

WLQs follow a different EW(CIV)-Lbol/Ledd relationship than normal quasars.

The weakness of emission lines in WLQs is due to low BLR covering factor, not a soft ionizing continuum.

The high-ionization to low-ionization line region ratio is lower in WLQs than in normal quasars.

Abstract

The nature of weak emission-line quasars (WLQs) is probed by comparing the Baldwin effect (BEff) in WLQs and normal quasars (QSOs). We selected 81 high-redshift (z>2.2) and 2 intermediate-redshift (z=1.66 and 1.89) WLQs. Their rest-frame equivalent widths (EWs) of the C IV emission-line and their Eddington ratio were obtained from the Sloan Digital Sky Survey Data Release 7 (SDSS DR7) Quasar Catalogue or from Diamond-Stanic et al. We compare the parameters of WLQs with these of 81 normal quasars from Bright Quasar Survey (BQS) and 155 radio-quiet and radio-intermediate quasars detected by SDSS and Chandra. The influence of the Eddington ratio, Lbol/Ledd, and the X-ray to optical luminosity ratio,alpha_ox, on the BEff is analysed. We find that WLQs follow a different relationship on the EW(CIV)-Lbol/Ledd plane than normal quasars. This relationship disagrees with the super-Eddington hypothesis. The weakness/absence of emission-lines in WLQs does not seem to be caused by their extremely soft ionizing continuum but by low covering factor (Omega) of their broad line region (BLR). Comparing emission-line intensities indicates that the ratios of high-ionization line and low-ionization line regions (i.e. Omega_(HIL)/Omega_(LIL)) are lower in WLQs than in normal QSOs. The covering factor of the regions producing C IV and Lyalpha emission-lines are similar in both WLQs and QSOs.