Lyman-Alpha Emission in Star-forming Galaxies: Low-Redshift Counterparts of Primeval Galaxies?

Abstract

We present UV plus optical spectra of three galaxies, Mrk 496, Mrk 357, Tol 1924-416, obtained by matching the size of the optical aperture with that of IUE. From the Hα/Hβ ratios, we estimate values of E(B - V)-extinction due to dust-for the two Markarian galaxies which are higher than those derived in previous works. We have used the new values of E(B - V) to deredden the Lyα fluxes, although this technique can be considered an approximation of the detailed extinction process, since it is unlikely that the dust is homogeneously distributed within a galaxy. An important aspect of our approach is the application of different extinction curves according to the different metallicity of the three galaxies, using the extinctions laws of our Galaxy, the Large Magellanic Cloud, and the Small Magellanic Cloud. The resulting Lyα/Hβ ratios are consistent with case B recombination (within the observational uncertainties) without invoking absorption of multiple scattered Lyα photons by dust. Since multiple scattering must be present because of the large H I column densities in our galaxies, we interpret our findings as an effect of the geometrical distribution of gas and dust inside the galaxies. The decreasing trend of observed Lyα/Hβ with metallicity reported in previous works result directly from extinction rather than multiple scattering. Our results suggest that the search for primeval galaxies using the Lyα line emission can be still considered a valid strategy.