LYMAN-ALPHA EMISSION FROM GALAXIES

We use new models of stellar population synthesis to compute the Lyalpha emission from galaxies with different star formation histories and initial mass functions. The models include all phases of stellar evolution and recent advances in the theories of stellar interiors and atmospheres. We find that dust-free galaxies would have Lyalpha equivalent widths of 50-200 angstrom, i.e., significantly higher than previous estimates, except from a few times 10(7) to 10(9) yr after a burst of star formation. We also consider several other factors that can affect the observed Lyalpha emission: the contributions by supernova remnants and active galactic nuclei, the orientation of a galaxy, and absorption by dust. We then use this framework to interpret the observations of and searches for Lyalpha emission from nearby star-forming galaxies, damped Lyalpha systems, blank sky, and the companions of quasars and damped Lyalpha systems. We suggest that, when Lyalpha emission is weak or absent, as is the case in most star-forming galaxies at low redshifts and in damped Lyalpha systems at high redshifts, the observed abundance of dust is sufficient to absorb most of the Lyalpha photons. On the other hand, when Lyalpha emission is strong, the presence of highly ionized species such as C IV and He II, large velocity widths, or nearby quasars indicate that much of the ionizing radiation may be supplied by active galactic nuclei. The null results of the many searches for Lyalpha emission from primeval galaxies are probably a consequence of the relatively brief periods in which galaxies are nearly dust-free and hence Lyalpha-bright.