The Very Large Telescope Lyman-Break Galaxy Redshift Survey – IV. Gas and galaxies at z ∼ 3 in observations and simulations

We use a combination of observations and simulation to study the relationship between star-forming galaxies and the intergalactic medium at z ≈ 3. The observed star-forming galaxy sample is based on spectroscopic redshift data taken from a combination of Very Large Telescope (VLT) Lyman-break galaxy (LBG) Redshift Survey (VLRS) data and Keck Low-Resolution Imaging Spectrometer (LRIS) observations in fields centred on bright background quasi-stellar objects (QSOs), whilst the simulation data is taken from the Galaxies–Intergalactic Medium Interaction Calculation (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, consistent with expectations from linear theory. At smaller scales, the root-mean-square (RMS) peculiar velocities in the simulation overpredict the difference between the simulated real- and z-space galaxy correlation functions. Peculiar velocity pairs with separations smaller than 1 h−1 Mpc have a smaller dispersion and explain the z-space correlation function better. The Lyα auto- and cross-correlation functions in the GIMIC simulation appear to show infall smaller than implied by the expected βLyα ≈ 1.3 (McDonald et al.). There is a possibility that the 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 Keck + VLRS correlation functions comes from the presence of velocity errors for the observed LBGs, which dominate at ≲ 1 h− 1 Mpc scales. When these are taken into account, the observed LBG correlation functions are well matched by the high amplitude of clustering, shown by higher mass (M* > 109 M⊙) galaxies in the simulation. The simulated cross-correlation function shows similar neutral gas densities around galaxies to those seen in the observations. The simulated and observed Lyα z-space autocorrelation functions again agree better with each other than with the βLyα ≈ 1.3 infall model. Our overall conclusion is that, at least in the simulation, gas and galaxy peculiar velocities are generally 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, in agreement with previous results.