Lyman-Alpha Emission from Galaxies

We use new models of stellar population synthesis to compute the Lyα 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 Lyα equivalent widths of 50-200 Å, i.e., significantly higher than previous estimates, except from a few times 107 to 109 yr after a burst of star formation. We also consider several other factors that can affect the observed Lyα 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 Lyα emission from nearby star-forming galaxies, damped Lyα systems, blank sky, and the companions of quasars and damped Lyα systems. We suggest that, when Lyα emission is weak or absent, as is the case in most star-forming galaxies at low redshifts and in damped Lyα systems at high redshifts, the observed abundance of dust is sufficient to absorb most of the Lyα photons. On the other hand, when Lyα emission is strong, the presence of highly ionized species such as C iv and He 11, 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 Lyα emission from primeval galaxies are probably a consequence of the relatively brief periods in which galaxies are nearly dust-free and hence Lyα-bright.