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 HII galaxies covering a wide range of metallicities and physical prop erties. We have found Lyar emission in 4 galaxies with blueshifted abs orption features, leading to P Cygni like profiles in 3 of them. In al l these objects the O I and Si II absorption lines are also blueshifte d 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 mor e. The rest of the sample shows broad damped Ly alpha absorption profi les 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 th e detectability of Ly alpha in emission. Relatively small column densi ties of neutral gas with even very small dust content would destroy th e Ly alpha emission if this gas is static with respect to the ionized region where Ly alpha photons originate. The situation changes dramati cally when most of the neutral gas is velocity-shifted with respect to the ionized regions because resonant scattering by neutral hydrogen w ill be most efficient at wavelengths shorter than the Ly alpha emissio n, allowing the Ly alpha photons to escape (at least partially). This mechanism complements the effect of porosity in the neutral interstell ar medium discussed by other authors, which allows to explain the esca pe of Ly alpha photons in regions surrounded by static neutral gas, bu t with only partial covering factors. The anisotropy of these gas flow s and their dependence on the intrinsic properties of the violent star -forming episodes taking place in these objects (age, strength, gas ge ometry,...) might explain (in part) the apparent lack of correlation b etween other properties (like metallicity) and the frequency of occure nce and strength of Ly alpha emission in star-forming galaxies. Attemp ts to derive the comoving star-formation rate at high redshifts from L y alpha emission searches are highly questionable.