Using the Properties of Broad Absorption Line Quasars to Illuminate Quasar Structure

TL;DR

This study investigates whether spectral properties can distinguish broad absorption line quasars from non-BAL quasars using statistical and machine learning methods, revealing minimal differences and challenging existing models.

Contribution

It demonstrates that observed spectral features cannot reliably differentiate BALQs from non-BALQs, and proposes an alternative disk-wind model consistent with these findings.

Findings

Spectral features show few significant differences between BALQ and non-BALQ samples.

Current narrow disk-wind models are inconsistent with observations.

An alternative disk-wind model with a wide opening angle and dense clumps is proposed.

Abstract

A key to understanding quasar unification paradigms is the emission properties of broad absorption line quasars (BALQs). The fact that only a small fraction of quasar spectra exhibit deep absorption troughs blueward of the broad permitted emission lines provides a crucial clue to the structure of quasar emitting regions. To learn whether it is possible to discriminate between the BALQ and non-BALQ populations given the observed spectral properties of a quasar, we employ two approaches: one based on statistical methods and the other supervised machine learning classification, applied to quasar samples from the Sloan Digital Sky Survey. The features explored include continuum and emission line properties, in particular the absolute magnitude, redshift, spectral index, line width, asymmetry, strength, and relative velocity offsets of high-ionisation CIV $\lambda 1549$ and low-ionisation MgII $\lambda 2798$ lines. We consider a complete population of quasars, and assume that the statistical distributions of properties represent all angles where the quasar is viewed without obscuration. The distributions of the BALQ and non-BALQ sample properties show few significant differences. None of the observed continuum and emission line features are capable of differentiating between the two samples. Most published narrow disk-wind models are inconsistent with these observations, and an alternative disk-wind model is proposed. The key feature of the proposed model is a disk-wind filling a wide opening angle with multiple radial streams of dense clumps.