Obscuration of Ly alpha photons in star-forming galaxies

We present a new study of the correlations between the Ly alpha emission, the UV extinction, and the metal content in a sample of 21 local, low-metallicity starburst galaxies from archival IUE spectra. We have consistently reextracted all the spectra using the optimal extraction algorithm by Kinney and coworkers, and we have also included galaxies not previously studied. In 40% of the cases our new measures of the Ly alpha equivalent width, W-alpha, differ from those reported in the literature by up to 50% of their value. The new measures show no significant correlation with either the obscuration of the UV continuum or the Balmer decrement, and only a very weak correlation with the metal index [O/H]. Using the flux ratio Ly alpha/H beta instead of W-alpha to take into account differences in the ionizing conditions of the nebular gas does not change these results. This shows that the extinction vicissitudes of the Ly alpha and nonresonant radiations have been decoupled during their propagations through the ISM. We interpret this as evidence that the ISM in the sample galaxies is, on average, highly inhomogeneous and that the transport of Ly alpha photons is primarily controlled by the ISM geometry rather than by the amount of dust. If the ISM geometry is mainly the result of the energy release from the star formation activity, we speculate that a similar phenomenology was also present at high redshifts. As the median of the absolute value of W-alpha in our sample is relatively large, the line can be efficiently used to measure the redshifts of primeval galaxy candidates at redshifts 2 less than or similar to z less than or similar to 7 via optical spectroscopy with the 10 m class telescopes.