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 Heii reionization are thought to be the primary cause of the large fluctuations observed in the Heii to Hi Lyα forest optical depth ratio, at z≃ 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 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 , the longer mean free path for photons with frequencies above and below the Heii ionization edge means these fluctuations have less effect on the Siiv and Civ ionization balance. Furthermore, UVB models which produce the largest fluctuations in specific intensity at the Heii 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 Instead, we find spatial variations in the IGM metallicity will dominate any scatter in Our results suggest that observational evidence for homogeneity in the observed 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 ionization corrections is likely intrinsic, and therefore provides a valuable constraint on intergalactic metal enrichment scenarios at these redshifts. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.