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Keywords:

  • galaxies: active;
  • quasars: emission lines;
  • ultraviolet: galaxies;
  • X-rays: galaxies

ABSTRACT

The nature of weak emission-line quasars (WLQs) is probed by comparing the Baldwin effect (BEff) in WLQs and normal quasars [quasi-stellar objects (QSOs)]. We selected 81 high-redshift (z > 2.2) and two 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 those of 81 normal quasars from Bright Quasar Survey 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, αox, on the BEff is analysed. We find that WLQs follow a different relationship on the EW(C iv)–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 (Ω/4π) of their broad-line region. Comparison of emission-line intensities indicates that the ratios of high-ionization line and low-ionization line regions (i.e. ΩHILLIL) are lower in WLQs than in normal QSOs. The covering factors of the regions producing C iv and Lyα emission lines are similar in both WLQs and QSOs.