  The nature of the interaction between galaxies and the intergalactic medium
(IGM) is one of the most fundamental problems in astrophysics. The accretion of
gas onto galaxies provides fuel for star formation, while galactic winds
transform the nearby IGM in a number of ways. One exciting technique to study
this gas is through the imaging of hydrogen Lyman-alpha emission. We use
cosmological simulations to study the Lyman-alpha signals expected from the
growth of cosmic structure from z=0-5. We show that if dust absorption is
negligible, recombinations following the absorption of stellar ionizing photons
dominate the total Lyman-alpha photon production rate. However, galaxies are
also surrounded by "Lyman-alpha coronae" of diffuse IGM gas. These coronae are
composed of a combination of accreting gas and material ejected from the
central galaxy by winds. The Lyman-alpha emission from this phase is powered by
a combination of gravitational processes and the photoionizing background.
While the former dominates at z~0, collisional excitation following
photo-heating may well dominate the total emission at higher redshifts. The
central regions of these systems are dense enough to shield themselves from the
metagalactic ionizing background; unfortunately, in this regime our simulations
are no longer reliable. We therefore consider several scenarios for the
emission from the central cores, including one in which self-shielded gas does
not emit at all. We show that the combination of star formation and cooling IGM
gas can explain most of the observed "Lyman-alpha blobs" at z~3, with the
important exception of the largest sources. On the other hand, except under the
most optimistic assumptions, cooling IGM gas cannot explain the observations on
its own.

Astrophysics