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

Abstract

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.