Double troughs in broad absorption line quasars and Ly-alpha-N V line-locking

Abstract

In a previous comparative study by Weymann et al. of the emission line and continuum properties of radio-quiet QSOs and broad absorption-line (BAL) QSOs, a difference spectrum, in the region of the C IV λ1549 trough, comprising the mean spectrum of 34 BALQSOs and the mean spectrum of 31 non-BALQSOs, surprisingly revealed double troughs separated in velocity space by the N v λ1240-Lyα splitting of ~ 5900 km s^(-1). In this contribution we investigate the reality of these features in a spectroscopic sample of72 BALQSOs consisting of the original 34 BALQSOs and an additional 38 BALQSOs. An atlas of the C IV BALs of72 BALQSOs is presented. We find that only 22% of this sample 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% level (as opposed to being the result of statistical fluctuations of BAL troughs occurring over random outflow velocities in a limited sample). We describe possible scenarios for their formation, involving the line-locking of Lyα-Nv within the BAL region. We show that no simple mechanism involving radiation pressure, alone can create the deep double troughs through the shadowing of N v λ1240 by Lyα. More complicated amplification mechanisms may be necessary. If real, the double-trough signature has the following important implications to our understanding of the BAL phenomenon: (1) The occurrence of broad absorption is somehow tied to an absolute velocity frame which is the same from quasar to quasar. (2) It follows then that the outflowing material within the BAL region is accelerating. (3) If line-locking is the mechanism behind the formation of the double troughs, then radiation pressure plays an important role in the dynamics of the BAL region.

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