A new sample of broad absorption-line quasars exhibiting the ghost of Lyman α

We have searched the broad absorption-line quasar (BAL QSO) sample presented recently by Reichard et al. for objects exhibiting the so-called ‘ghost of Lyman α’. This ghost manifests as a hump near −5900 km s−1 in the troughs of the broad absorption lines and provides strong evidence for the importance of line driving in powering the outflows from BAL QSOs. Of the 224 sample BAL QSOs selected from the Sloan Digital Sky Survey Early Data Release, 198 satisfy our redshift constraints and 58 show clear evidence of multiple-trough (MT) structure in the C iv 1550 Å line. A composite spectrum constructed from this MT sample already shows evidence for a ghost feature. Narrowing our classification scheme further, we define a set of 36 objects that individually show evidence of a ghost feature, and then apply further cuts to arrive at a final ‘best sample’ that contains our seven strongest ghost candidates. A further five objects show evidence for a ghost feature that is almost strong enough to merit inclusion in our best sample. Despite its limited size, our best sample more than doubles the number of known BAL QSOs with clear ghost signatures and should make an excellent basis for detailed follow-up studies.