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

Abstract

Quasar accretion-disk outflows might play an important role in galaxy\nevolution, but they are notoriously difficult to study due to line saturation\nand blending problems in the Ly-alpha forest. We circumvent these problems by\nconstructing median composite spectra of diverse broad absorption lines (BALs)\nand `mini-BALs' in SDSS-III BOSS quasars at redshifts 2.3 < z < 3.5. Sorting by\nCIV 1549,1551 absorption-line strength with AlIII 1855,1863 as an additional\nindicator of low ionisations (LoBALs) we find the following: (1) Deeper and\nbroader BALs are accompanied by weaker HeII 1640 emission lines, consistent\nwith softer ionising spectra producing more effective radiative acceleration.\n(2) PV 1118,1128 absorption is present with resolved ~1:1 depth ratios in all\ncomposites from mini-BALs to strong BALs indicating that line saturation, large\ntotal column densities log N_H(cm^-2) > 22.7, and large ionisation parameters\nlog U > -0.5 are common. (3) Different observed depths in saturated lines\nidentify inhomogeneous partial covering on spatial scales <0.006 pc, where\nweak/low-abundance transitions like PV form in small high-column density clumps\nwhile stronger/broader lines like CIV form in larger volumes. (4) Excited-state\nSiIV* 1073 and CIII* 1176 lines in BAL outflows indicate typical densities n_e\n> 3 x 10^5 cm^-3 and maximum radial distances R < 23pc from the quasars. (5)\nFor reasonable actual distances, the median BAL outflow has minimum kinetic\nenergy L_K/L > 0.005(R/1.2pc) sufficient (by some estimates) for feedback to\ngalaxy evolution. (6) LoBAL quasars have the largest median outflow column\ndensities, highest velocities, and weakest HeII 1640 emission in our study;\nthey appear to be at one extreme in a distribution of quasar properties where\nsofter ionising spectra drive more powerful outflows.\n