Abstract
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.
Highlights
High-resolution quasar absorption line spectroscopy has enabled the statistical detection of intergalactic metals at cosmic overdensities as low as = ρ/ ρ = 1–10 at z 3
Scannapieco et al (2006) and Pieri et al (2006) noted this scatter may be attributable to spatial variations in the metallicity of the intergalactic medium (IGM), they did not rule out the possibility that spatial fluctuations in the ultraviolet background (UVB) spectral shape may play a role
We carefully examine the impact of the UVB fluctuations on the Si IV and C IV
Summary
These fluctuations are thought to arise from the small number of quasars lying within the short (He II ionizing photon) mean free path expected at the tail-end of He II reionization (Fardal et al 1998; Bolton et al 2006; Furlanetto 2009a), small scale radiative transfer effects (Maselli & Ferrara 2005; Tittley & Meiksin 2007), collisionally ionized gas (Muzahid, Srianand & Petitjean 2011) or a significant number of thermally broadened lines (Fechner & Reimers 2007) may produce column density ratios with η ≤ 10 These spatial inhomgeneities in the UVB spectral shape should have an impact on the ionization balance of the metals in the IGM.. All atomic data are taken from Morton (2003), solar abundances are from Asplund et al (2009) and all distances are given in comoving units
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