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We study the $z\approx3.5$ intergalactic medium (IGM) by comparing new,high-quality absorption spectra of eight QSOs with $\langle z_{\rm QSO}\rangle=3.75$, to virtual observations of the EAGLE cosmological hydrodynamicalsimulations. We employ the pixel optical depth method and uncover strongcorrelations between various combinations of HI, CIII, CIV, SiIII, SiIV, andOVI. We find good agreement between many of the simulated and observedcorrelations, including OVI(HI). However, the observed median optical depthsfor the CIV(HI) and SiIV(HI) relations are higher than those measured from themock spectra. The discrepancy increases from up to $\approx0.1$ dex at$\tau_{\rm HI}=1$ to $\approx1$ dex at $\tau_{\rm HI}=10^2$, where we arelikely probing dense regions at small galactocentric distances. As possiblesolutions, we invoke (a) models of ionizing radiation softened above 4 Ryd toaccount for delayed completion of HeII reionization; (b) simulations run at ahigher resolution; (c) the inclusion of additional line broadening due tounresolved turbulence; and (d) increased elemental abundancess; however, noneof these factors can fully explain the observed differences. Enhancedphotoionization of HI by local sources, which was not modelled, could offer asolution. However, the much better agreement with the observed OVI(HI)relation, which we find probes a hot and likely collisionally-ionized gasphase, indicates that the simulations are not in tension with the hot phase ofthe IGM, and suggests that the simulated outflows may entrain insufficient coolgas.

Observations of metals in the $z\approx3.5$ intergalactic medium and comparison to the EAGLE simulations