High-redshift galaxies and the Lyα forest in a cold dark matter universe

We use a cosmological hydrodynamic simulation of a cold dark matter universe to investigate theoretically the relationship between high-redshift galaxies and the Lyalpha forest at redshift z = 3. Galaxies in the simulation are surrounded by halos of hot gas, which nevertheless contain enough neutral hydrogen to cause a Lyalpha flux decrement, its strength increasing with galaxy mass. A comparison with recent observational data by Adelberger et al. on the Lyalpha forest around galaxies reveals that actual galaxies may have systematically less Lyalpha absorption within 1 Mpc of them than our simulated galaxies. In order to investigate this possibility, we add several simple prescriptions for galaxy feedback on the intergalactic medium (IGM) to the evolved simulation. These include the effect of photoionizing background radiation coming from galactic sources, galactic winds whose only effect is to deposit thermal energy into the IGM, and another, kinetic model for winds, which are assumed to evacuate cavities in the IGM around galaxies. We find that only the latter is able to produce a large effect, enough to match the tentative observational data, given the energy available from star formation in the simulated galaxies. Another intriguing possibility is that a selection effect is responsible, so that galaxies with low Lyalpha absorption are preferentially included in the sample. This is also viable but predicts galaxy properties (including clustering) that are very different from the other scenarios.