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, focussing primarily on the gas
outside the virial radius, motivated by recent HI 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 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 percent 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.
