The impact of spatial fluctuations in the ultraviolet background on intergalactic carbon and silicon

Spatial inhomogeneities in the spectral shape of the ultraviolet background (UVB) at the tail-end of He II reionization are thought to be the primary cause of the large fluctuations observed in the He II to H I Ly alpha forest optical depth ratio, tau(He) (II)/tau(H) (I), at z similar or equal to 2-3. These spectral hardness fluctuations will also influence the ionization balance of intergalactic metals; we extract realistic quasar absorption spectra from a large hydrodynamical simulation to examine their impact on intergalactic Si IV and C IV 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(He) (II)/tau(H) (I), the longer mean free path for photons with frequencies above and below the He II ionization edge means these fluctuations have less effect on the Si IV and C IV ionization balance. Furthermore, UVB models which produce the largest fluctuations in specific intensity at the He II ionization edge also have the softest ionizing spectra, and thus result in photoionization rates which are too low to produce significant fluctuations in the observed tau(Si) (IV)/tau(C) (IV). Instead, we find spatial variations in the IGM metallicity will dominate any scatter in tau(Si) (IV)/tau(C) (IV). Our results suggest that observational evidence for homogeneity in the observed tau(Si) (IV)/tau(C) (IV) distribution does not rule out the possibility of significant fluctuations in the UVB spectral shape at z similar or equal to 2-3. On the other hand, the scatter in metallicity inferred from observations of intergalactic C IV and Si IV absorption at z similar or equal to 2-3 using spatially uniform ionization corrections is likely intrinsic, and therefore provides a valuable constraint on intergalactic metal enrichment scenarios at these redshifts.