Quasar emission lines as probes of orientation: implications for disc wind geometries and unification

Abstract

The incidence of broad absorption lines (BALs) in quasar samples is often interpreted in the context of a geometric unification model consisting of an accretion disc and an associated outflow. We use the the Sloan Digital Sky Survey (SDSS) quasar sample to test this model by examining the equivalent widths (EWs) of CIV 1550Å, Mg II 2800Å, [OIII] 5007Å and C III] 1909Å. We find that the emission line EW distributions in BAL and non-BAL quasars are remarkably similar -- a property that is inconsistent with scenarios in which a BAL outflow rises equatorially from a geometrically thin, optically thick accretion disc. We construct simple models to predict the distributions from various geometries; these models confirm the above finding and disfavour equatorial geometries. We show that obscuration, line anisotropy and general relativistic effects on the disc continuum are unlikely to hide an EW inclination dependence. We carefully examine the radio and polarisation properties of BAL quasars. Both suggest that they are most likely viewed (on average) from intermediate inclinations, between type 1 and type 2 AGN. We also find that the low-ionization BAL quasars in our sample are not confined to one region of `Eigenvector I' parameter space. Overall, our work leads to one of the following conclusions, or some combination thereof: (i) the continuum does not emit like a geometrically thin, optically thick disc; (ii) BAL quasars are viewed from similar angles to non-BAL quasars, i.e. low inclinations; (iii) geometric unification does not explain the fraction of BALs in quasar samples.

Accepted to MNRAS. 16 pages, 11 figures

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Cited by in corpus (11)

• Physical properties of 15 quasars at $z\gtrsim 6.5$
• Monte Carlo Radiative Transfer
• Stratified disc wind models for the AGN broad-line region: ultraviolet, optical and X-ray properties
• Testing AGN outflow and accretion models with CIV and HeII emission line demographics in z=2 quasars
• Radiation hydrodynamics models of Active Galactic Nuclei: Beyond the central parsec
• A disc wind model for blueshifts in quasar broad emission lines
• The Kinematics of Quasar Broad Emission Line Regions Using a Disk-wind Model
• Offsets between X-ray and Radio Components in X-ray Jets: The AtlasX
• SIROCCO: A Publicly Available Monte Carlo Ionization and Radiative Transfer Code for Astrophysical Outflows
• [OIII] emission in z=2 quasars with and without Broad Absorption Lines
• A Giant Metrewave Radio Telescope Survey of Radio-loud Broad Absorption Line Quasars