[O III] Equivalent Width and Orientation Effects in Quasars

TL;DR

This study demonstrates that the equivalent width of [OIII] in quasars correlates with inclination angle, confirming a disc-like optical emission structure and enabling inclination-based corrections for luminosity estimates.

Contribution

It provides the first empirical evidence linking [OIII] equivalent width to quasar inclination and explores the geometry of emission regions in quasars.

Findings

EW([OIII]) distribution matches models with random inclination angles

EW([OIII]) can serve as an inclination proxy for quasars

Broad line regions show no inclination dependence, indicating a disc-like geometry

Abstract

The flux of the [OIII] 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 theta between the disc axis and the line of sight. Based on this, the equivalent width (EW) of [OIII] should provide a direct measure of theta. Here we analyze the distribution of EW([OIII]) in a sample of ~6,000 SDSS 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([OIII]) can be used as a proxy of the inclination, to correct the measured continuum emission and then estimate the bolometric luminosity of quasars; 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 (Hbeta, Mg II, C IV) provides no evidence of inclination effects, suggesting a disc-like geometry of the broad emission line region.