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.