tokens: interact gravitationally and radiative cooling is left on. This density threshold is set to 10 per cent of the star formation threshold, which is ρ SF = 0.13 cm −3 for the arepo simulations and ρ SF = 0.33 cm −3 for the Sherwood simulation. By looking at volume-weighted spherically averaged density profiles of haloes, we estimate that these density thresholds correspond to an average distance of 0.7 R vir for the arepo simulations and 0.4 R vir for the Sherwood simulation. There is of course some scatter between individual haloes, however. The Illustris, HVEL and FAST simulations all include AGN feedback, but the Sherwood simulation does not include black holes. We do not however expect this to change our results significantly [see, for example, Tescari et al. ( 2011 ) and Suresh et al. ( 2015 ), who find that AGN are not very important for metal enrichment at high redshift]. 2.1.3 UV background Illustris, HVEL and FAST were all run using the Faucher-Giguère et al. ( 2009 ) model of the UV background. A correction for the self-shielding of gas from the UV background is also incorporated into these simulations, based on the Rahmati et al. ( 2013 ) model; however, this is only included below z = 6. The Sherwood simulation was run using the Haardt Madau ( 2012 ) UV background, which turns on at z = 15. Note that throughout this work, unless specified, we used the Haardt Madau ( 2012 ) model of the UV background for computing the ionization state of the gas. The original UV background of the simulations will still be important when considering, for example, the temperature of the gas in the simulations. We do investigate the effect of changing the shape and amplitude of the UV background on the metal-line absorbers we model in Section 4.2 . Ideally, we would use a more realistic inhomogeneous UV background, such as in Finlator et al. ( 2015 ), but it is reassuring in that work they find that there is only a factor of ∼2 difference between the neutral fractions calculated using their model and the Haardt Madau ( 2012 ) background. 2.2 Feedback and the high-<italic>z</italic> IGM/CGM Galactic feedback is generally invoked to reconcile the shapes of the dark matter halo mass function and galactic stellar mass function (e.g. White Frenk 1991 ). For our purposes, the feedback is important both as a means of regulating the number of stars produced (and hence the amount of metals) and also for distributing these metals throughout the IGM. In the left-hand panel of Fig. 1 , we show the galaxy stellar mass function of the different simulations at z = 6 for haloes resolved by at least 100 dark matter particles. Here we compare to observations of the stellar mass functions at z ∼ 6 (Stark et al. 2009 ; González et al. 2011 ) but note that the Illustris run has already been compared with observations across a wide range of redshifts z = 0–7 in Genel et al. ( 2014 ). The HVEL model shows an increase in galaxies with low stellar masses, as the constant fast
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