On the structure and energetics of quasar broad absorption-line outflows

Abstract

Quasar accretion-disc 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 α 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 C IV λλ1549,1551 absorption-line strength with Al III λλ1855,1863 as an additional indicator of low ionizations (LoBALs) we find the following: (1) Deeper and broader BALs are accompanied by weaker He II λ1640 emission lines, consistent with softer ionizing spectra producing more effective radiative acceleration. (2) P V λλ1118,1128 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 NH(cm−2) 22.7, and large ionization 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 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∗ λ1073 and C III∗ λ1176 lines in BAL outflows indicate typical densities ne 3 × 105 cm−3 and maximum radial distances R 23 pc from the quasars. (5) For reasonable actual distances, the median BAL outflow has minimum kinetic energy LK/L 0.005(R/1.2 pc) sufficient for feedback to galaxy evolution. (6) LoBAL quasars have the largest median outflow column densities, highest velocities, and weakest He II λ1640 emission in our study; they appear to be at one extreme in a distribution of quasar properties where softer ionizing spectra drive more powerful outflows.