The impact of spatial fluctuations in the ultra-violet background on
  intergalactic carbon and silicon
  Spatial inhomogeneities in the spectral shape of the ultra-violet background
(UVB) at the tail-end of HeII reionisation are thought to be the primary cause
of the large fluctuations observed in the HeII to HI Ly-a forest optical depth
ratio, tau_HeII/tau_HI, at z~2-3. These spectral hardness fluctuations will
also influence the ionisation balance of intergalactic metals; we extract
realistic quasar absorption spectra from a large hydrodynamical simulation to
examine their impact on intergalactic SiIV and CIV absorbers. Using a variety
of toy UVB models, we find that while the predicted spatial inhomogeneities in
spectral hardness have a significant impact on tau_HeII/tau_HI, the longer mean
free path for photons with frequencies above and below the HeII ionisation edge
means these fluctuations have less effect on the SiIV and CIV ionisation
balance. Furthermore, UVB models which produce the largest fluctuations in
specific intensity at the HeII ionisation edge also have the softest ionising
spectra, and thus result in photo-ionisation rates which are too low to produce
significant fluctuations in the observed tau_SiIV/tau_CIV. Instead, we find
spatial variations in the IGM metallicity will dominate any scatter in
tau_SiIV/tau_CIV. Our results suggest that observational evidence for
homogeneity in the observed tau_SiIV/tau_CIV distribution does not rule out the
possibility of significant fluctuations in the UVB spectral shape at z~2-3. On
the other hand, the scatter in metallicity inferred from observations of
intergalactic CIV and SiIV absorption at z~2-3 using spatially uniform
ionisation corrections is likely intrinsic, and therefore provides a valuable
constraint on intergalactic metal enrichment scenarios at these redshifts.
