Gas around galaxy haloes: methodology comparisons using hydrodynamical simulations of the intergalactic medium

We perform cosmological simulations of the intergalactic medium (IGM) at redshift z ˜ 3 using the numerical gravity-hydrodynamics codes GADGET-3 and ENZO for the purpose of modelling the gaseous environments of galaxies. We identify haloes in the simulations using three different algorithms. Different rank orderings of the haloes by mass result, introducing a limiting factor, in identifying haloes with observed galaxies. We also compare the physical properties of the gas between the two codes, focusing primarily on the gas outside the virial radius, motivated by recent H I absorption measurements of the gas around z ˜ 2-3 galaxies. The internal dispersion velocities of the gas in the haloes have converged for a box size of 30 comoving Mpc, but the centre-of-mass peculiar velocities of the haloes have not up to a box size of 60 comoving Mpc. The density and temperature of the gas within the instantaneous turn-around radii of the haloes are adequately captured for box sizes of 30 Mpc on a side, but the results are highly sensitive to the treatment of unresolved, rapidly cooling gas, with the gas mass fraction within the virial radius severely depleted by star formation in the GADGET-3 simulations. Convergence of the gas peculiar velocity field on large scales requires a box size of at least 60 Mpc. Outside the turn-around radius, the physical state of the gas agrees to 30 per cent or better both with box size and between simulation methods. We conclude that generic IGM simulations make accurate predictions for the intergalactic gas properties beyond the halo turn-around radii, but the gas properties on smaller scales are highly dependent on star formation and feedback implementations.