ISDC Data Centre for Astrophysics

The environment of Weak Emission-Line Quasars

We analyse the ultraviolet and X-ray properties of 83 weak emission-line quasars (WLQ) and ~240 ordinary quasars to answer whether the Baldwin effect is responsible for the unusual weakness of emission-lines in WLQ or not. We find that this weakness is rather caused by a low covering factor of the broad line region in WLQ than by their extremely soft ionizing continuum.

Nikolajuk & Walter, 2011, accepted in MNRAS

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. (2009). 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, L_Bol/L_Edd, and the X-ray to optical luminosity ratio, α_ox, on the BEff is analysed. We find that WLQs follow a different relationship on the EW(CIV)–L_Bol/L_Edd 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 (Ω/4π) 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. Ω_HIL/Ω_LIL) are lower in WLQs than in normal QSOs. The covering factor of the regions producing C IV and Ly α emission-lines are similar in both WLQs and QSOs.

Equivalent width of C IV versus the accretion ratio
Filled blue squares show 81 BQS quasars analysed by Baskin & Laor (2004). Filled red triangles and upper limits refer to 76 WLQs taken from Shen et al. (2011) or Diamond-Stanic et al. (2009). Points with error bars refer to objects with a significance of EW higher than 5σ. Otherwise, upper limits are shown. Dashed line is the best linear fit to BQS quasars.
Credits: ISDC/Nikolajuk

Rest-frame spectrum of a weak-line quasar
The Rest-frame spectrum of the WLQ SDSS J170108.89+395443.0 binned and corrected for Galactic reddening is compared to a composite spectrum of normal quasars.
Credits: ISDC/Nikolajuk

Rest frame equivalent width of C IV emission-line versus the spectral index α_ox.
Solid blue and open cyan squares refers to Type 1 non-BAL and BAL quasars, respectively. Those 155 radio-quiet and radio-intermediate sources are taken from Green et al. (2009). Solid red triangles and upper limits show 12 WLQ objects. Star denotes NLS1 PHL 1811. Solid line is the relationship obtained by Wu et al.(2009).
Credits: ISDC/Nikolajuk