The z=0.54 LoBAL Quasar SDSS J085053.12+445122.5: II. The Nature of
  Partial Covering in the Broad-Absorption-Line Outflow
  It has been known for 20 years that the absorbing gas in broad absorption
line quasars does not completely cover the continuum emission region, and that
partial covering must be accounted for to accurately measure the column density
of the outflowing gas. However, the nature of partial covering itself is not
understood. Extrapolation of the SimBAL spectral synthesis model of the HST COS
UV spectrum from SDSS J0850+4451 reported by Leighly et al. 2018 to
non-simultaneous rest-frame optical and near-infrared spectra reveals evidence
that the covering fraction has wavelength dependence, and is a factor of 2.5
times higher in the UV than in the optical and near-infrared bands. The
difference in covering fraction can be explained if the outflow consists of
clumps that are small and either structured or clustered relative to the
projected size of the UV continuum emission region, and have a more diffuse
distribution on size scales comparable to the near-infrared continuum emission
region size. The lower covering fraction over the larger physical area results
in a reduction of the measured total column density by a factor of 1.6 compared
with the UV-only solution. This experiment demonstrates that we can compare
rest-frame UV and near-infrared absorption lines, specifically HeI*10830, to
place constraints on the uniformity of absorption gas in broad absorbing line
quasars.
