The average absorption properties of broad absorption line quasars at 800 < lambda_rest < 3000 {\AA}, and the underlying physical parameters

TL;DR

This study investigates the physical parameters influencing broad absorption line quasars by analyzing their spectral properties across 800-3000 Å, revealing key factors like ionization potential, continuum slope, and emission line strength that affect absorption features and quasar orientation.

Contribution

It provides a comprehensive analysis of BALQ spectral properties over a broad wavelength range and identifies specific physical parameters controlling absorption profiles and quasar viewing angles.

Findings

No detectable Lyman edge in BAL gas.

Higher ionization correlates with increased absorption depth.

He II EW and spectral slope influence C IV BAL velocity and depth.

Abstract

Broad absorption line quasars (BALQs) present a large diversity in their broad absorption line (BAL) profiles. To investigate what physical parameters underlie this diversity, we construct a sample of BALQs which covers lambda_rest ~ 800-3000 {\AA}, based on the Sloan Digital Sky Survey DR7 quasar catalogue. The average BAL properties are evaluated by taking the ratios of average BALQ spectra to the average spectra of matched samples of non-BALQs, where the matching is based on various emission properties. We find the following properties. (i) There is no detectable Lyman edge associated with the BAL absorbing gas (tau<0.1). (ii) The known increase of average absorption depth with the ionization potential extends to the higher ionization N V and O VI BALs. We also find that the C IV BAL profile is controlled by two parameters. (i) The He II emission EW, which controls the typical velocity of the C IV BAL, but does not affect the absorption depth. (ii) The spectral slope in the 1700-3000 {\AA} range (alpha_UVl), which controls the C IV peak absorption depth, but does not affect the typical velocity. The He II EW and alpha_UVl also control the observed fraction of quasars that are BALQs. We suggest that a lower He II EW may indicate a weaker ionizing continuum, which allows the outflow to reach higher velocities before being overionized, possibly without a need to invoke a shielding gas. A redder continuum may indicate a more inclined system, and a larger covering factor and larger column of the outflow along the line of sight.