Tracing the Reionization-Epoch Intergalactic Medium with Metal Absorption Lines

TL;DR

This study uses high-resolution simulations to analyze metal absorption lines in the intergalactic medium during reionization, revealing insights into early feedback, enrichment sources, and ionization states at z>6.

Contribution

It introduces detailed cosmological simulations including galactic outflows to study metal-line observability and properties during reionization, contrasting different ionization models.

Findings

Simulations broadly fit observed z~5-6 metal-line data.

O I absorbers indicate neutral IGM patches at z~6.

C IV traces IGM metals effectively at z~5-6.

Abstract

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 reionization. We run high-resolution cosmological simulations including galactic outflows to study the observability and physical properties of 5 ions (C II, C IV, O I, Si II, Si IV) in absorption between z=8->5. We apply three cases for ionization conditions: Fully neutral, fully reionized, and a patchy model based on the flux from the nearest galaxy. We find that our simulations 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. (2006) suggest significant neutral IGM patches down to z~6. Strong C IV absorbers at z~6 may be the result of ionization by their parent galaxy. Our outflows have typical speeds of ~200 km/s and mass loading factors of ~6. Such high mass loading is critical for enriching the IGM to the observed levels while curtailing star formation to match the observed z~6 rest-frame UV luminosity function. The volume filling factor of metals increases during this epoch, but only reaches ~1% for Z>10^(-3) Zsolar by z=5. 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 Omega(C IV) from z=8->5, in contrast to its relative constancy from z=5->2. Our simulations do not support widespread early IGM enrichment from e.g. Pop III stars. High-z absorbers arise from metals on their first outward journey from galaxies, at distances less than 50 kpc. The galaxies responsible for early IGM enrichment have typical M*=10^(7.0-8.5) Msolar.