The average absorption properties of broad absorption line quasars at 800 < λrest < 3000 A, 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 approximate to 800 angstrom, 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 (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 equivalent width (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 A 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.