The VLT LBG Redshift Survey - IV. Gas and galaxies at z ~ 3 in
  observations and simulations
  We use observations and simulation to study the relationship between
star-forming galaxies and the intergalactic medium at z~3. The observed galaxy
sample is based on spectroscopic redshift data from a combination of the VLT
LBG Redshift Survey and Keck observations in fields centred on bright z>3 QSOs,
whilst the simulation data is taken from GIMIC. In the simulation, we find that
the dominant peculiar velocities are in the form of large-scale coherent
motions of gas and galaxies. Gravitational infall of galaxies towards one
another is also seen. At smaller scales, the peculiar velocities in the
simulation over-predict the difference between the simulated real- and z-space
galaxy correlation functions. Peculiar velocity pairs separated by <1Mpc/h have
a smaller dispersion and explain the z-space correlation function better. The
Ly{\alpha} auto- and cross-correlation functions in the GIMIC simulation show
infall smaller than implied by previous work. This reduced infall may be due to
the galaxy wide outflows implemented in the simulation. The main challenge in
comparing these simulated results with the observed correlation functions comes
from the presence of velocity errors for the observed LBGs which dominate at
~1Mpc/h scales. When these are taken into account, the observed LBG correlation
function is well matched by a simulated $M_*>10^9M_\odot$ galaxy sample. The
simulated cross-correlation shows similar neutral gas densities around galaxies
as are seen in the observations. The simulated and observed Ly{\alpha} z-space
autocorrelation functions agree well with each other. Our overall conclusion is
that gas and galaxy peculiar velocities are towards the low end of expectation.
Finally, little direct evidence is seen in either simulation or observations
for high transmission near galaxies due to feedback. (Abridged)
