Low-z MgII Broad Absorption-Line Quasars from the Sloan Digital Sky
  Survey
  We present a sample of 68 low-z MgII low-ionization broad absorption-line
(loBAL) quasars. The sample is uniformly selected from the SDSS5 according to
the following criteria: (1) 0.4<z<0.8, (2) median S/N>7, and (3) MgII
absorption-line width > 1600 \kms. The last criterion is a trade-off between
the completeness and consistency with respect to the canonical definition of
BAL quasars that have the `balnicity index' BI>0 in CIV BAL. We adopted such a
criterion to ensure that ~90% of our sample are classical BAL quasars and the
completeness is ~80%, based on extensive tests using high-z quasar samples with
measurements of both CIV and MgII BALs. We found (1) MgII BAL is more
frequently detected in quasars with narrower Hbeta emission-line, weaker [OIII]
emission-line, stronger optical FeII multiplets and higher luminosity. In term
of fundamental physical parameters of a black hole accretion system, loBAL
fraction is significantly higher in quasars with a higher Eddington ratio than
those with a lower Eddington ratio. The fraction is not dependent on the black
hole mass in the range concerned. The overall fraction distribution is broad,
suggesting a large range of covering factor of the absorption material. (2)
[OIII]-weak loBAL quasars averagely show undetected [NeV] emission line and a
very small line ratio of [NeV] to [OIII]. However, the line ratio in non-BAL
quasars, which is much larger than that in [OIII]-weak loBAL quasars, is
independent of the strength of the [OIII] line. (3) loBAL and non-loBAL quasars
have similar colors in near-infrared to optical band but different colors in
ultraviolet. (4) Quasars with MgII absorption lines of intermediate width are
indistinguishable from the non-loBAL quasars in optical emission line
properties but their colors are similar to loBAL quasars, redder than non-BAL
quasars. We also discuss the implication of these results.
