Lyman- alpha Emission in Star-forming Galaxies

Abstract

We report IUE observations of five blue, low-metallicity, star-forming galaxies sufficiently redshifted to permit detection of Lyα. Two of the galaxies exhibit Lyα emission at modest levels far below the recombination limit; two objects have Lyα in absorption. These observations confirm and extend earlier work which indicates that relatively unevolved galaxies do not exhibit striking Lyα emission signatures, even when strong ultraviolet continuum emission from hot stellar populations is observed. In the complete sample of blue galaxies observed at Lyα (13 objects), there is evidence for the correlation of Lyα emission with metallicity expected if Lyα were destroyed by dust absorption. This correlation suggests that primeval galaxies could have significant Lyα emission if they have extremely low metallicity. Extrapolation of these results to primeval galaxies remains uncertain; the escape of Lyα depends upon the history of metal enrichment and upon both the spatial and velocity distribution of the gas. We suggest that trapping of Lyα in H II regions surrounded by large amounts of neutral material is responsible for the destruction of a substantial fraction of the Lyα flux in normal star-forming galaxies. Dust absorption affects Lyα line emission more than the UV continuum. Near-infrared continuum searches for primeval galaxies might thus be more effective than searches at Lyα.