Gas around galaxy haloes - II: hydrogen absorption signatures from the
  environments of galaxies at redshifts 2 < z < 3
  We compare predictions of large-scale cosmological hydrodynamical simulations
for neutral hydrogen absorption signatures in the vicinity of 1e11 - 1e12.5
MSun haloes with observational measurements. Two different hydrodynamical
techniques and a variety of prescriptions for gas removal in high density
regions are examined. Star formation and wind feedback play only secondary
roles in the HI absorption signatures outside the virial radius, but play
important roles within. Accordingly, we identify three distinct gaseous regions
around a halo: the virialized region, the mesogalactic medium outside the
virial radius arising from the extended haloes of galaxies out to about two
turnaround radii, and the intergalactic medium beyond. Predictions for the
amount of absorption from the mesogalactic and intergalactic media are robust
across different methodologies, and the predictions agree with the amount of
absorption observed around star-forming galaxies and QSO host galaxies.
Recovering the measured amount of absorption within the virialized region,
however, requires either a higher dynamic range in the simulations, additional
physics, or both.
