ON THE LYMAN-ALPHA EMISSION OF STARBURST GALAXIES

Nearby starburst galaxies have consistently shown anomalous Lyalpha/Hbeta ratios. By reanalyzing the published IUE/optical observations, we show that most starbursts present a normal Lyalpha emission, consistent with case B recombination theory, provided extinction laws appropriate to their metallicities are used. This implies that extinction is more important than multiple resonant scattering effects. The anomalous emission and absorption lines present in a few remaining galaxies are simply explained if they are observed in the postburst phase, between about 10(7) and 10(8) yr after the start of the burst. We use updated stellar population synthesis models to show that anomalous ratios are produced by the aging of stellar populations, since the underlying stellar Lyalpha line is important in the cooler massive stars. The inferred low duty cycle of massive star formation accounts naturally for the failure to detect large numbers of Lyalpha-emitting galaxies in deep surveys and at high redshift. Some testable predictions of the proposed scenario are also discussed.