Quasar accretion-disk outflows might play an important role in galaxy evolution , but they are notoriously difficult to study due to line saturation and blending problems in the Ly \alpha forest .
We circumvent these problems by constructing median composite spectra of diverse broad absorption lines ( BALs ) and ‘ mini-BALs ’ in SDSS-III BOSS quasars at redshifts 2.3 \leq z \leq 3.5 .
Sorting by C iv \lambda 1549,1551 absorption-line strength with Al iii \lambda 1855,1863 as an additional indicator of low ionisations ( LoBALs ) we find the following : ( 1 ) Deeper and broader BALs are accompanied by weaker He ii \lambda 1640 emission lines , consistent with softer ionising spectra producing more effective radiative acceleration .
( 2 ) P v \lambda 1118,1128 absorption is present with resolved \sim 1:1 depth ratios in all composites from mini-BALs to strong BALs indicating that line saturation , large total column densities \log N _ { H } ( \textrm { cm } ^ { -2 } ) \gtrsim 22.7 , and large ionisation parameters \log U \gtrsim - 0.5 are common .
( 3 ) Different observed depths in saturated lines identify inhomogeneous partial covering on spatial scales \lesssim 0.006 pc , where weak/low-abundance transitions like P v form in small high-column density clumps while stronger/broader lines like C iv form in larger regions .
( 4 ) Excited-state S iv * \lambda 1073 and C iii * \lambda 1176 lines in BAL outflows indicate typical densities n _ { e } \gtrsim 3 \times 10 ^ { 5 } cm ^ { -3 } and maximum radial distances R \lesssim 23 pc from the quasars .
( 5 ) For reasonable actual distances , the median BAL outflow has minimum kinetic energy L _ { K } / L \gtrsim 0.005 ( R / 1.2 \textrm { pc } ) sufficient for feedback to galaxy evolution .
( 6 ) LoBAL quasars have the largest median outflow column densities , highest velocities , and weakest He ii \lambda 1640 emission in our study ; they appear to be at one extreme in a distribution of quasar properties where softer ionising spectra drive more powerful outflows .

Key words : quasars : absorption lines – quasars : general