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