We investigate the metallicity evolution and metal content of the intergalactic medium ( IGM ) and galactic halo gas from z = 2 \rightarrow 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 ( WHIM ) 6.4 times more .
Despite these significant differences , all wind models produce metal-line statistics within a factor of two of existing observations .
This is because O vi , C iv , Si iv , and Ne viii absorbers primarily arise from T < 10 ^ { 5 } K , photo-ionised gas that is enriched to similar levels in the three feedback schemes . O vi absorbers trace the diffuse phase with \rho / \bar { \rho } \la 100 , which is enriched to \sim 1 / 50 { Z } _ { \odot } at z = 0 , although the absorbers themselves usually exceed 0.3 { Z } _ { \odot } and arise from inhomogeneously distributed , un-mixed 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 \sim 10 ^ { 4 } K gas inside haloes ( \rho / \bar { \rho } \ga 100 ) , although there appear to be too many C iv absorbers relative to observations .
We predict COS will observe a population of Ne viii photo-ionised absorbers tracing T < 10 ^ { 5 } K , \rho / \bar { \rho } \sim 10 gas with equivalent widths of 10-20 mÅ . Mg x and Si xii are rarely detected in COS S/N=30 simulated sight lines ( dn / dz \ll 1 ) , although simulated Si xii detections trace halo gas at T = 10 ^ { 6 - 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 ionisation species tracing lower-density , older metals .
At z = 0 , the 90 % of baryons outside of galaxies are enriched to \bar { Z } = 0.096 { Z } _ { \odot } , but the 65 % of unbound baryons in the IGM have \bar { Z } = 0.018 { Z } _ { \odot } and contain only 4 % of all metals , a large decline from 20 % at z = 2 , because metals from early winds often re-accrete onto galaxies while later winds are less likely to escape their haloes .
We emphasise 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 with other similar publications .