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