A Comprehensive Study of Broad Absorption Line Quasars: I. Prevalence of
  HeI* Absorption Line Multiplets in Low-Ionization Objects
  Neutral Helium multiplets, HeI*3189,3889,10830 are very useful diagnostics to
the geometry and physical conditions of the absorbing gas in quasars. So far
only a handful of HeI* detections have been reported. Using a newly developed
method, we detected HeI*3889 absorption line in 101 sources of a well-defined
sample of 285 MgII BAL quasars selected from the SDSS DR5. This has increased
the number of HeI* BAL quasars by more than one order of magnitude. We further
detected HeI*3189 in 50% (52/101) quasars in the sample. The detection fraction
of HeI* BALs in MgII BAL quasars is about 35% as a whole, and increases
dramatically with increasing spectral signal-to-noise ratios, from 18% at S/N
<= 10 to 93% at S/N >= 35. This suggests that HeI* BALs could be detected in
most MgII LoBAL quasars, provided spectra S/N is high enough. Such a
surprisingly high HeI* BAL fraction is actually predicted from photo-ionization
calculations based on a simple BAL model. The result indicates that HeI*
absorption lines can be used to search for BAL quasars at low-z, which cannot
be identified by ground-based optical spectroscopic survey with commonly seen
UV absorption lines. Using HeI*3889, we discovered 19 BAL quasars at z<0.3 from
available SDSS spectral database. The fraction of HeI* BAL quasars is similar
to that of LoBAL objects.
