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.
Highlights
The Universe undergoes its last major transition at z ∼ 6 as the first stars and early galaxies finish the process of reionizing the previously neutral intergalactic medium (IGM)
The other observables we have considered such as Ω(C iv) all deal with column densities, which should remain unaffected by turbulent broadening as long as lines are not saturated. b-parameters are the observable most likely affected by turbulent broadening, and may explain some of the broad C iv over N (C iv) profiles seen by Ryan-Weber et al (2006) and Simcoe (2006), Becker et al (2009) suggests that these may be multi-component systems with smaller line widths if observed at higher resolution
We explore high redshift metal-line absorbers using a state-of-the-art, 2 × 5123 particle Gadget-2 cosmological simulation enriching the IGM with star formation-driven galactic outflows
Summary
The Universe undergoes its last major transition at z ∼ 6 as the first stars and early galaxies finish the process of reionizing the previously neutral intergalactic medium (IGM). Oppenheimer & Dave (2006, hereafter OD06) explored C iv absorbers in cosmological hydrodynamic simulations, employing various heuristic prescriptions for galactic outflows to enrich the z ∼ 2−6 IGM They determined that prescriptions based on momentum-driven winds were best able to reproduce C iv observations. In this paper we consider the above questions by studying z = 5 − 8 metal-line absorbers in cosmological hydrodynamic simulations that incorporate our favored momentum-driven wind model In this wind model, the outflow speed and mass loss rate are tied to galaxy mass using a heuristic prescription as expected for momentumdriven winds (Murray, Quatert, & Thompson 2005). Throughout we use Asplund et al (2005) abundances when calibrating metallicities relative to solar
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