The average absorption properties of broad absorption line quasars at 800 < λrest < 3000 Å, and the underlying physical parameters

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 λ_rest ≈ 800-3000 Å, based on the Sloan Digital Sky Survey Data Release 7 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 (τ < 0.1). (ii) The known increase of average absorption depth with the ionization potential extends to the higher ionization NV and OVI BALs. We also find that the C IV BAL profile is controlled by two parameters. (i) The He II emission equivalent width (EW), which controls the typical velocity of the CIV BAL, but does not affect the absorption depth. (ii) The spectral slope in the 1700-3000 Å range (α_UVl), which controls the C IV peak absorption depth, but does not affect the typical velocity. The He II EW and α_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.