Broad Absorption Line Quasars with Redshifted Troughs: High-Velocity
  Infall or Rotationally Dominated Outflows?
  We report the discovery in the Sloan Digital Sky Survey and the SDSS-III
Baryon Oscillation Spectroscopic Survey of seventeen broad absorption line
(BAL) quasars with high-ionization troughs that include absorption redshifted
relative to the quasar rest frame. The redshifted troughs extend to velocities
up to v=12,000 km/s and the trough widths exceed 3000 km/s in all but one case.
Approximately 1 in 1000 BAL quasars with blueshifted C IV absorption also has
redshifted C IV absorption; objects with C IV absorption present only at
redshifted velocities are roughly four times rarer. In more than half of our
objects, redshifted absorption is seen in C II or Al III as well as C IV,
making low-ionization absorption at least ten times more common among BAL
quasars with redshifted troughs than among standard BAL quasars. However, the C
IV absorption equivalent widths in our objects are on average smaller than
those of standard BAL quasars with low-ionization absorption. We consider
several possible ways of generating redshifted absorption. The two most likely
possibilities may be at work simultaneously, in the same objects or in
different ones. Rotationally dominated outflows seen against a quasar's
extended continuum source can produce redshifted and blueshifted absorption,
but variability consistent with this scenario is seen in only one of the four
objects with multiple spectra. The infall of relatively dense and
low-ionization gas to radii as small as 400 Schwarzschild radii can in
principle explain the observed range of trough profiles, but current models do
not easily explain the origin and survival of such gas. Whatever the origin(s)
of the absorbing gas in these objects, it must be located at small radii to
explain its large redshifted velocities, and thus offers a novel probe of the
inner regions of quasars.
