On the Lyman-Alpha Emission of Starburst Galaxies

Nearby starburst galaxies have consistently shown anomalous Ly-alpha/H-betaratios. By re-analysing the published IUE/optical observations, we show thatmost starbursts present a normal Ly-alpha emission, consistent with case Brecombination theory, provided extinction laws appropriate to theirmetallicities are used. This implies that extinction is more important thanmultiple resonant scattering effects. The anomalous emission and absorptionlines present in a few remaining galaxies are simply explained if they areobserved in the post-burst phase, between about 10$^7$ and 10$^8$ yrs after thestart of the burst. We use updated stellar population synthesis models to showthat anomalous ratios are produced by the aging of stellar populations, sincethe underlying stellar Ly-alpha line is important in the cooler massive stars.The inferred low-duty cycle of massive star formation accounts naturally forthe failure to detect large numbers of Ly-alpha--emitting galaxies in deepsurveys and at high redshift. Some testable predictions of the proposedscenario are also discussed.Comment: 7 PostScript pages with 4 Figures (included), astro-ph/yymmnn