A COMPREHENSIVE STUDY OF BROAD ABSORPTION LINE QUASARS. I. PREVALENCE OF He I* ABSORPTION LINE MULTIPLETS IN LOW-IONIZATION OBJECTS

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