Neutral gas properties of Lyman continuum emitting galaxies: Column densities and covering fractions from UV absorption lines

Context. The processes allowing the escape of ionizing photons from galaxies into the intergalactic medium are poorly known.
Aims: To understand how Lyman continuum (LyC) photons escape galaxies, we constrain the H I covering fractions and column densities using ultraviolet (UV) H I and metal absorption lines of 18 star-forming galaxies that have Lyman series observations. Nine of these galaxies are confirmed LyC emitters.
Methods: We fit the stellar continuum, dust attenuation, metal, and H I properties to consistently determine the UV attenuation, as well as the column densities and covering factors of neutral hydrogen and metals. We used synthetic interstellar absorption lines to explore the systematics of our measurements. Then we applied our method to the observed UV spectra of low-redshift and z 3 galaxies.
Results: The observed H I lines are found to be saturated in all galaxies. An indirect approach using O I column densities and the observed O/H abundances yields H I column densities of log(N_{H I}) 18.6-20 cm^-2. These columns are too high to allow the escape of ionizing photons. We find that the known LyC leakers have H I covering fractions less than unity. Ionizing photons escape through optically thin channels in a clumpy interstellar medium. Our simulations confirm that the H I covering fractions are accurately recovered. The Si II and H I covering fractions scale linearly, in agreement with observations from stacked Lyman break galaxy spectra at z 3. Thus, with an empirical correction, the Si II absorption lines can also be used to determine the H I coverage. Finally, we show that a consistent fitting of dust attenuation, continuum, and absorption lines is required to properly infer the covering fraction of neutral gas and subsequently to infer the escape fraction of ionizing radiation.
Conclusions: These measurements can estimate the LyC escape fraction, as we demonstrate in a companion paper.