Tracing the re-ionization-epoch intergalactic medium with metal absorption lines

Intergalactic medium (IGM) metal absorption lines observed in z ≳ 6 spectra offer the opportunity to probe early feedback processes, the nature of enriching sources and the topology of re-ionization. We run high-resolution cosmological simulations including galactic outflows to study the observability and physical properties of five ions (C ii, C iv, O i, Si ii, Si iv) in absorption between z = 8 and 5. We apply three cases for ionization conditions: fully neutral, fully re-ionized and a patchy model based on the flux from the nearest galaxy. We find that our simulations can broadly fit available z ∼ 5-6 IGM metal-line data, although all observations cannot be accommodated with a single ionization condition. Variations in O i absorbers among sight lines seen by Becker et al. suggest significant neutral IGM patches down to z ∼ 6. Strong C iv absorbers at z ∼ 6 may be the result of ionization by the galaxy responsible for that enrichment, although the identification of the neighbouring galaxy will have to wait to confirm this. Our outflows have typical speeds of ∼200 km s^-1 and mass loading factors of ∼6. Such high mass loading is critical for enriching the IGM to the observed levels while sufficiently curtailing early star formation to match the observed rest-frame ultraviolet luminosity function. The volume filling factor of metals increases during this epoch, but only reaches ∼1 per cent for Z > 10^-3 Z_⊙ by z = 5. Detectable absorbers generally trace inhomogeneously distributed metals residing outside of galactic haloes. C iv is an ideal tracer of IGM metals at z ∼ 5-6, with dropping global ionization fractions to either higher or lower redshifts. This results in a strongly increasing global C iv mass density from z = 8 to 5, in contrast to its relative constancy from z = 5 to 2. Our simulations do not support widespread early IGM enrichment from e.g. Population III stars, as this would overpredict the numbers of weak C iv absorbers in the latest data. High-z absorbers arise from metals mostly on their first outward journey, at distances 5-50 physical kpc, and often exhibit broad profiles (δv > 200 km s^-1) as a result of outflowing peculiar velocities in the strongest systems. The galaxies responsible for early IGM enrichment have typical stellar masses of 10 ^7.0-8.5 M_⊙, and star formation rates ≲1 M _⊙ yr^-1. Future facilities will be able to study the high-z galaxy-absorber connection in detail, revealing a wealth of information about feedback processes in the re-ionization epoch.