HST study of Lyman-alpha emission in star-forming galaxies: the effect of neutral gas flows

We present high dispersion HST GHRS UV spectroscopic observations of 8 H II galaxies covering a wide range of metallicities and physical properties. We have found Lyalpha \ emission in 4 galaxies with blueshifted absorption features, leading to P Cygni like profiles in 3 of them. In all these objects the O I and Si II absorption lines are also blueshifted with respect to the ionized gas, indicating that the neutral gas is outflowing in these galaxies with velocities up to 200 km s(-1) or more. The rest of the sample shows broad damped Lyalpha \ absorption profiles centered at the wavelength corresponding to the redshift of the H II emitting gas. We therefore find that the velocity structure of the neutral gas in these galaxies is the driving factor that determines the detectability of Lyalpha \ in emission. Relatively small column densities of neutral gas with even very small dust content would destroy the Lyalpha \ emission if this gas is static with respect to the ionized region where Lyalpha \ photons originate. The situation changes dramatically when most of the neutral gas is velocity-shifted with respect to the ionized regions because resonant scattering by neutral hydrogen will be most efficient at wavelengths shorter than the Lyalpha \ emission, allowing the Lyalpha \ photons to escape (at least partially). This mechanism complements the effect of porosity in the neutral interstellar medium discussed by other authors, which allows to explain the escape of Lyalpha \ photons in regions surrounded by static neutral gas, but with only partial covering factors. The anisotropy of these gas flows and their dependence on the intrinsic properties of the violent star-forming episodes taking place in these objects (age, strength, gas geometry,...) might explain (in part) the apparent lack of correlation between other properties (like metallicity) and the frequency of occurence and strength of Lyalpha \ emission in star-forming galaxies. Attempts to derive the comoving star-formation rate at high redshifts from Lyalpha \ emission searches are highly questionable. Based on observations with the NASA/ESA {\it Hubble Space Telescope