Aligned metal absorbers and the ultraviolet background at the end of
  reionization
  We use observations of spatially-aligned C II, C IV, Si II, Si IV, and O I
absorbers to probe the slope and intensity of the ultraviolet background (UVB)
at $z \sim 6$. We accomplish this by comparing observations with predictions
from a cosmological hydrodynamic simulation using three trial UVBs applied in
post-processing: a spectrally soft, fluctuating UVB calculated using
multi-frequency radiative transfer; a soft, spatially-uniform UVB; and a hard,
spatially-uniform "quasars-only" model. When considering our paired
high-ionization absorbers (C IV/Si IV), the observed statistics strongly prefer
the hard, spatially-uniform UVB. This echoes recent findings that cosmological
simulations generically underproduce strong C IV absorbers at $z>5$. A single
low/high ionization pair (Si II/Si IV), by contrast, shows a preference for the
HM12 UVB, while two more (C II/C IV and O I/C IV) show no preference for any of
the three UVBs. Despite this, future observations of specific absorbers,
particularly Si IV/C IV, with next-generation telescopes probing to lower
column densities should yield tighter constraints on the UVB.
