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

Abstract

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 , , and absorbers primarily arise from T < 105 K, photoionized gas that is enriched to similar levels in the three feedback schemes. absorbers trace the diffuse phase with , which is enriched to ∼1/50 Z⊙ at z= 0, although the absorbers themselves usually exceed 0.3 Z⊙ and arise from inhomogeneously distributed, unmixed winds. Turbulent broadening is required to match the observed equivalent width and column density statistics for . and absorbers trace primarily T∼ 104 K gas inside haloes (), although there appear to be too many absorbers relative to observations. We predict the COS will observe a population of photoionized absorbers tracing T < 105 K, gas with equivalent widths of 10–20 mÅ. and are rarely detected in COS signal‐to‐noise ratio 30 simulated sight‐lines (dn/dz≪ 1), although simulated detections trace halo gas at T= 106–107 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 , but 65 per cent of unbound baryons in the IGM have 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.

Cosmic distribution of highly ionized metals and their physical conditions in the EAGLE simulations

“…Column density distribution functions of Civ (top) and Ovi (bottom) at z ≈ 0 (left) and z ≈ 2 (right) in simulations performed by different groups. Solid, dashed and dotted curves show the results presented in this work, in Oppenheimer et al (2012) and in Bird et al (2015), respectively. The shaded area around the dashed curve shows the 1-σ poisson error associated with the reported CDDFs in Oppenheimer et al (2012).…”

“…However, the effects of non-equilibrium ionization and local sources of ionizing radiation on the distribution of different ions may not be negligible (e.g., Schaye 2006;Oppenheimer & Schaye 2013a,b;Rahmati et al 2013b) Modelling the transport of metals self-consistently from star forming regions into the IGM through galactic outflows is challenging. Simulations often do not capture the evolution of outflows self-consistently, e.g., because they temporarily turn off radiative cooling (e.g., Theuns et al 2002;Shen et al 2010Shen et al , 2013 and/or decouple the outflows hydrodynamically (e.g., Oppenheimer & Davé 2006Oppenheimer et al 2012;Vogelsberger et al 2014), mainly to increase the feedback efficiency and to improve the numerical convergence. Even simulations which generate outflows more self-consistently require subgrid models and those prescriptions come with parameters whose values require calibration (e.g., Schaye et al 2015).…”

Star formation sustained by gas accretion

“…Accordingly, the metallicity of the web increases with decreasing redshift, reaching the level of 10 −2 Z at z = 0 (see Fig. 3, and also Oppenheimer et al 2012).…”

The circumgalactic medium of high-redshift galaxies

“…Generally, one might expect that the shape of the relation depends somewhat on feedback prescriptions (Oppenheimer et al 2012). However, we note that feedback has been accurately calibrated in P14 to reproduce globally averaged galactic properties.…”