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tokens: give rise to metal absorption in gaseous haloes. Simcoe et al. ( 2006 ) estimated absorbers sizes around z ∼ 2.3 galaxies ranging from the sub-parsec to the kiloparsec. This is consistent with the transverse scales of C iv absorbers derived in Rauch et al. ( 2001 ) from multisightline analysis of lensed QSOs at sub-kpc scales (see also Petitjean et al. 2000 , based on Mg II). Based on photoionization modelling of C iv absorbers at similar redshifts, Schaye, Carswell Kim ( 2007 ) derived typical densities of n H ∼ 10 −3.5 cm −3 with sizes of ∼100 pc. Similarly, Crighton et al. ( 2015 ) observe metal absorption lines arising near a z ≃ 2.5 galaxy and derive a (100–500) pc size constraint (see also Arrigoni Battaia et al. 2015 ), which they argue is a ubiquitous size-scale for all low-ion halo gas in the circumgalactic environments of both low and high redshift galaxies. Pieri et al. ( 2014 ) conclude that a typical low-ionization absorber associated with the CGM of strong Lyman α forest systems at z ∼ 2.5 is ∼30 pc in size. Furthermore, the gas giving rise to various absorption lines may also have unexpected geometries with little resemblance to spherical clouds (Churchill et al. 2015 ). Obtaining ∼100 pc hydrodynamic resolution at the virial radius by simply running higher resolution simulations of isolated haloes will be prohibitively expensive. Following the observed scaling of the gas resolution in the halo, we would obtain a mean r cell ≃ 0.1 kpc physical at z = 2 with a L14-class simulation. That is, gas mass resolution approaching 30 M ⊙ , requiring ≳ 10 10 gas cells in a 10 12 M ⊙ halo. This is not a realistic goal for the near future. Even given such a simulation, the minimum size of resolved clouds would be a few times larger than the size of individual resolution elements. On the other hand, numerical methods may enable novel ways to focus resolution within the halo regime. If we can avoid clustering the computational effort in the densest regions of space – within galaxies themselves – much higher spatial resolutions can be achieved in lower density media. In adaptive mesh refinement simulations, the refinement criterion can be chosen as desired, on the gradient of the neutral hydrogen fraction for instance (Rosdahl Blaizot 2012 ), or with a progressively refined uniform Eulerian grid which is volume filling (Miniati 2014 ). In our moving mesh simulations, we can enforce a target gas cell mass criterion of any type through adaptive refinement and de-refinement of Voronoi cells. In future work we will explore this possibility as a solution to the resolution issue, in addition to quantifying the rates and modes of accretion using the tracer particle information. 7 CONCLUSIONS In this first paper of the series, we present a suite of high-resolution cosmological hydrodynamic zoom-in simulations targeted at understanding the properties of halo gas and cosmological accretion in ∼10 12 M ⊙ haloes at z = 2. Using the moving-mesh code arepo