The Soft, Fluctuating UVB at $z\sim6$ as Traced by C IV, SiIV, and CII
  The sources that drove cosmological reionization left clues regarding their
identity in the slope and inhomogeneity of the ultraviolet ionizing background
(UVB): Bright quasars (QSOs) generate a hard UVB with predominantly large-scale
fluctuations while Population II stars generate a softer one with smaller-scale
fluctuations. Metal absorbers probe the UVB's slope because different ions are
sensitive to different energies. Likewise, they probe spatial fluctuations
because they originate in regions where a galaxy-driven UVB is harder and more
intense. We take a first step towards studying the reionization-epoch UVB's
slope and inhomogeneity by comparing observations of 12 metal absorbers at
$z\sim6$ versus predictions from a cosmological hydrodynamic simulation using
three different UVBs: a soft, spatially-inhomogeneous "galaxies+QSOs" UVB; a
homogeneous "galaxies+QSOs" UVB (Haardt & Madau 2012); and a QSOs-only model.
All UVBs reproduce the observed column density distributions of CII, SiIV, and
CIV reasonably well although high-column, high-ionization absorbers are
underproduced, reflecting numerical limitations. With upper limits treated as
detections, only a soft, fluctuating UVB reproduces both the observed SiIV/CIV
and CII/CIV distributions. The QSOs-only UVB overpredicts both CIV/CII and
CIV/SiIV, indicating that it is too hard. The Haardt & Madau (2012) UVB
underpredicts CIV/SiIV, suggesting that it lacks amplifications near galaxies.
Hence current observations prefer a soft, fluctuating UVB as expected from a
predominantly Population II background although they cannot rule out a harder
one. Future observations probing a factor of two deeper in metal column density
will distinguish between the soft, fluctuating and QSOs-only UVBs.
