[O iii] equivalent width and orientation effects in quasars

• Published in: Monthly notices of the Royal Astronomical Society Vol. 411; no. 4; pp. 2223 - 2229
• Main Authors: Risaliti, G., Salvati, M., Marconi, A.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.03.2011; Wiley-Blackwell; Oxford University Press
• Subjects: Accretion disks; Astronomy; Earth, ocean, space; Equivalence; Exact sciences and technology; galaxies: active; Orientation; Quasars; quasars: general; galaxies: active; quasars: general; Continuum; Active galaxies; Quasars; Equivalent width; Luminosity; Broad line region; Random distribution; Accretion disks; Sky surveys; H-beta lines
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2010.17843.x

The flux of the [O iii]λ5007-Å line is considered to be a good indicator of the bolometric emission of quasars. The observed continuum emission from the accretion disc should instead be strongly dependent on the inclination angle θ between the disc axis and the line of sight. Based on this, the equivalent width (EW) of [O iii] should provide a direct measure of θ. Here we analyse the distribution of EW([O iii]) in a sample of ∼6000 Sloan Digital Sky Survey quasars, and find that it can be accurately reproduced assuming a relatively small intrinsic scatter and a random distribution of inclination angles. This result has several implications: (1) it is a direct proof of the disc-like emission of the optical continuum of quasars; (2) the value of EW([O iii]) can be used as a proxy of the inclination, to correct the measured continuum emission and then estimate the bolometric luminosity of quasars and (3) the presence of almost edge-on discs among broad-line quasars implies that the accretion disc is not aligned with the circumnuclear absorber, and/or that the covering fraction of the latter is rather small. Finally, we show that a similar analysis of EW distributions of broad lines (Hβ, Mg ii and C iv) provides no evidence of inclination effects, suggesting a disc-like geometry of the broad emission line region.