The z=0.54 LoBAL Quasar SDSS J085053.12+445122.5: II. The Nature of Partial Covering in the Broad-Absorption-Line Outflow

Abstract

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 the 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. Analysis of the spectral energy distribution and an HST near-infrared image shows that the contribution of the host galaxy to the near-infrared continuum is negligible. Observed absorption line variability trends show that it is probably not responsible for the effect either. 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. This experiment demonstrates that we can compare rest-frame UV and near-infrared absorption lines, specifically HeI*10830, to place constraints on the geometry of absorption gas in broad absorption line quasars.