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

We investigate the metallicity evolution and metal content of the intergalactic medium (IGM) and galactic halo gas from z = 2 to 0 using 110-million-particle cosmological hydrodynamic simulations. We focus on the detectability and physical properties of ultraviolet 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 6.4 times more. Despite these significant differences, all wind models produce metal-line statistics within a factor of 2 of existing observations. This is because O VI, C IV, Si IV and Ne VIII absorbers primarily arise from T < 10(5) K, photoionized gas that is enriched to similar levels in the three feedback schemes. O VI absorbers trace the diffuse phase with rho/<(rho)over bar> less than or similar to 100, which is enriched to similar to 1/50 Z(circle dot) at z = 0, although the absorbers themselves usually exceed 0.3 Z(circle dot) and arise from inhomogeneously distributed, unmixed winds. Turbulent broadening is required to match the observed equivalent width and column density statistics for O VI. C IV and Si IV absorbers trace primarily T similar to 10(4) K gas inside haloes (rho/(rho) over bar greater than or similar to 100), although there appear to be too many C IV absorbers relative to observations. We predict the COS will observe a population of Ne VIII photoionized absorbers tracing T < 10(5) K, rho/<(rho)over bar> similar to 10 gas with equivalent widths of 10-20 m angstrom. Mg X and Si XII are rarely detected in COS signal-to-noise ratio 30 simulated sight-lines (dn/dz << 1), although simulated Si XII detections trace halo gas at T = 10(6)-10(7) K. 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 ionization species tracing lower density, older metals. At z = 0, 90 per cent of baryons outside galaxies are enriched to (Z) over bar = 0.096 Z(circle dot), but 65 per cent of unbound baryons in the IGM have (Z) over bar = 0.018 Z(circle dot) and contain only 4 per cent of all metals, a large decline from 20 per cent at z = 2, because metals from early winds often re-accrete on to galaxies while later winds are less likely to escape their haloes. We emphasize that our results are sensitive to how metal mixing is treated in the simulations, and argue that the lack of mixing in our scheme may be the largest difference from other similar publications.