The intergalactic medium over the last 10 billion years II: Metal-line absorption and physical conditions

TL;DR

This study uses cosmological simulations to analyze the metal enrichment and physical conditions of the intergalactic medium over the last 10 billion years, focusing on UV metal-line absorbers observable with Hubble's COS.

Contribution

It demonstrates the necessity of galactic superwinds for IGM enrichment and compares different wind models' effects on metal distribution and absorption statistics.

Findings

Galactic superwinds are essential for IGM metal enrichment.

All wind models produce similar metal-line statistics within a factor of two of observations.

Different ions trace distinct temperature and density phases of the IGM.

Abstract

We investigate the metallicity evolution and content of the intergalactic medium (IGM) and galactic halo gas from z=2->0 using 110-million particle cosmological hydrodynamic simulations. We focus on the detectability and physical properties of UV resonance metal-line absorbers observable with Hubble's Cosmic Origins Spectrograph (COS). We confirm that galactic superwind outflows are required to enrich the IGM to observed levels down to z=0 using three wind prescriptions contrasted to a no-wind simulation. Our favoured momentum-conserved wind prescription deposits metals closer to galaxies owing to its moderate energy input, while the more energetic constant wind model enriches the warm-hot IGM 6.4x more. Despite these significant differences, all wind models produce metal-line statistics within a factor of two of existing observations. This is because OVI, CIV, SiIV, and NeVIII absorbers primarily arise from T<10^5 K, photo-ionised gas that is enriched to similar levels in the three feedback schemes. OVI absorbers trace the diffuse phase with delta<100, which is enriched to ~1/50 Zsol at z=0, although the absorbers themselves usually exceed 0.3 Zsol and arise from inhomogeneously distributed, un-mixed winds. CIV and SiIV absorbers trace primarily T~10^4 K gas inside haloes. We predict COS will observe a population of NeVIII photo-ionised absorbers tracing T<10^5 K, delta~10 gas. MgX and SiXII are rarely detected in COS S/N=30 simulated sight lines although detections trace T=10^(6-7) K halo gas. In general, the IGM is enriched in an outside-in manner, where wind-blown metals released at higher redshift reach lower overdensities, resulting in higher ionisation species tracing lower-density, older metals. At z=0, the 90% of baryons outside of galaxies are enriched to 0.096 Zsol, but the 65% of unbound baryons in the IGM have 0.018 Zsol and contain only 4% of all metals.