  We use a cosmological hydrodynamic simulation of a cold dark matter universe
to investigate theoretically the relationship between high redshift galaxies
and the Lyman=alpha 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 Ly-alpha flux decrement, its strength increasing with
galaxy mass. A comparison with recent observational data by Adelberger et. al
on the Ly-alpha forest around galaxies reveals that actual galaxies may have
systematically less Ly-alpha 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 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
  Ly-alpha absorption are preferentially included in the sample. This is also
viable, but predicts very different galaxy properties (including clustering)
than the other scenarios.
