Searching for the signature of radiative line driving: on the absence of Lyα-N v line-locking features in a large sample of BALQSOs

We have searched the hybrid broad absorption line quasar (BALQSO) catalogue of Scaringi et al. derived from Data Release 5 of the Sloan Digital Sky Survey 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. These criteria include significant broad absorption, strong intrinsic Ly alpha emission, narrow Ly alpha, strong N v absorption and a weak far-ultraviolet continuum. No single ghost candidate meets all of these criteria. Furthermore, we 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 difference 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, with comparable numbers appearing both redward and blueward of the ghost zone. Thus, the majority of objects identified here as strong ghost candidates are likely multitrough interlopers whose absorption feature simply brackets the region of interest.