Searching for the signature of radiative line driving: On the absence of
  Ly-alpha-N V line-locking features in a large sample of BALQSOs
  We have searched the hybrid BALQSO catalogue of Scaringi et al. derived from
DR5 of the SDSS in order to compile the largest sample of objects displaying
spectral signatures which may be indicative of radiative line driving. The
feature in question is the "ghost of Ly-alpha", a line-locking feature
previously identified in the broad C IV and Si IV absorption lines of a small
fraction of BALQSOs, and formed via the interaction of Ly-alpha photons with N
V ions. We test, where possible the criteria required to produce an observable
ghost feature and find that these criteria are not met significantly more often
in ghost-candidates than in a comparison sample chosen to exhibit relatively
featureless broad absorption troughs. Indeed, the only significant differences
we find between our ghost-candidate and comparison samples, is that on average,
our ghost-candidate sample displays (i) significantly stronger N V absorption,
and (ii) the onset of absorption occurs at lower velocities in our
ghost-candidate objects. Significantly, we find no evidence for an excess of
objects whose absorption troughs bracket the location of the Ly-alpha-N V
line-locking region, rather the location of ghost-like features appears to be
independent of any systematic velocity. Thus, the majority of objects
identified here as strong ghost-candidates are likely multi-trough interlopers
whose absorption feature simply bracket the region of interest.
