DOUBLE TROUGHS IN BROAD ABSORPTION-LINE QUASARS AND LY-ALPHA-N-V LINE-LOCKING

In a previous comparative study by Weymann et al. of the emission line and continuum properties of radio-quiet QSOs and broad absorption-lin e (BAL) QSOs, a difference spectrum, in the region of the C IV lambda1 549 trough, comprising the mean spectrum of 34 BALQSOs and the mean sp ectrum of 31 non-BALQSOs, surprisingly revealed double troughs separat ed in velocity space by the N V lambda1240-Lyalpha splitting of approx imately 5900 km s-1. In this contribution we investigate the reality o f these features in a spectroscopic sample of 72 BALQSOs consisting of the original 34 BALQSOs and an additional 38 BALQSOs. An atlas of the C IV BALs of 72 BALQSOs is presented. We find that only 22% of this s ample explicitly exhibits the double trough feature. When present the double troughs are deep. A Monte Carlo simulation of the mean C IV BAL suggests that the double-trough feature is real at only the 95%-98% l evel (as opposed to being the result of statistical fluctuations of BA L troughs occurring over random outflow velocities in a limited sample ). We describe possible scenarios for their formation, involving the l ine-locking of Lyalpha-N V within the BAL region. We show that no simp le mechanism involving radiation pressure alone can create the deep do uble troughs through the shadowing of N V lambda1240 by Lyalpha. More complicated amplification mechanisms may be necessary. If real, the do uble-trough signature has the following important implications to our understanding of the BAL phenomenon: (1) The occurrence of broad absor ption is somehow tied to an absolute velocity frame which is the same from quasar to quasar. (2) It follows then that the outflowing materia l within the BAL region is accelerating. (3) If line-locking is the me chanism behind the formation of the double troughs, then radiation pre ssure plays an important role in the dynamics of the BAL region.