  Identifying the mechanisms driving the escape of Lyman Continuum (LyC)
photons is crucial to find Lyman Continuum Emitter (LCE) candidates. To
understand the physical properties involved in the leakage of LyC photons, we
investigate the connection between the HI covering fraction, HI velocity width,
the Lyman alpha (LyA) properties and escape of LyC photons in a sample of 22
star-forming galaxies including 13 LCEs. We fit the stellar continua, dust
attenuation, and absorption lines between 920 and 1300 A to extract the HI
covering fractions and dust attenuation. Additionally, we measure the HI
velocity widths of the optically thick Lyman series and derive the LyA
equivalent widths (EW), escape fractions (fesc), peak velocities and fluxes at
the minimum of the LyA profiles. Overall, we highlight strong correlations
between the presence of low HI covering fractions and (1) low LyA peak
velocities; (2) more flux at the profile minimum; and (3) larger EW(LyA),
fesc(LyA), and fesc(LyC). Hence, low column density channels are crucial ISM
ingredients for the leakage of LyC and LyA photons. Additionally, galaxies with
narrower HI absorption velocity widths have higher LyA equivalent widths,
larger LyA escape fractions, and lower LyA peak velocity separations. This
suggests that these galaxies have low HI column density. Finally, we find that
dust regulates the amount of LyA and LyC radiation that actually escapes the
ISM. Overall, the ISM porosity is one origin of strong LyA emission and enables
the escape of ionizing photons in low-z leakers. However, this is not enough to
explain the largest fesc(LyC) observed, which indicates that the most extreme
LCEs are likely density-bounded along all lines of sight to the observer.
Overall, the neutral gas porosity constrains a lower limit to the escape
fraction of LyC and LyA photons, providing a key estimator of the leakage of
ionizing photons.
