On the structure and energetics of quasar broad absorption-line outflows
  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 < z < 3.5. Sorting by
CIV 1549,1551 absorption-line strength with AlIII 1855,1863 as an additional
indicator of low ionisations (LoBALs) we find the following: (1) Deeper and
broader BALs are accompanied by weaker HeII 1640 emission lines, consistent
with softer ionising spectra producing more effective radiative acceleration.
(2) PV 1118,1128 absorption is present with resolved ~1:1 depth ratios in all
composites from mini-BALs to strong BALs indicating that line saturation, large
total column densities log N_H(cm^-2) > 22.7, and large ionisation parameters
log U > -0.5 are common. (3) Different observed depths in saturated lines
identify inhomogeneous partial covering on spatial scales <0.006 pc, where
weak/low-abundance transitions like PV form in small high-column density clumps
while stronger/broader lines like CIV form in larger volumes. (4) Excited-state
SiIV* 1073 and CIII* 1176 lines in BAL outflows indicate typical densities n_e
> 3 x 10^5 cm^-3 and maximum radial distances R < 23pc from the quasars. (5)
For reasonable actual distances, the median BAL outflow has minimum kinetic
energy L_K/L > 0.005(R/1.2pc) sufficient (by some estimates) for feedback to
galaxy evolution. (6) LoBAL quasars have the largest median outflow column
densities, highest velocities, and weakest HeII 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.
