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

Abstract

Neutral helium multiplets, He i∗ 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∗ 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∗ 3189 in 50% (52/101) of the quasars in the sample. The detection fraction of He i∗ BALs in Mg ii BAL quasars is ∼35% as a whole, and it increases dramatically with increasing spectral signal-to-noise ratio (S/N), from ∼18% at S/N 10 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∗ λ3889, 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.