  The physical origin of low escape fractions of ionizing radiation derived
from massive star-forming galaxies at $z\sim3$-$4$ is not well understood. We
perform idealised disc galaxy simulations to understand how galactic properties
such as metallicity and gas mass affect the escape of Lyman Continuum (LyC)
photons using radiation-hydrodynamic simulations with strong stellar feedback.
We find that the luminosity-weighted escape fraction from a metal-poor
($Z=0.002$) galaxy embedded in a halo of mass $M_h\simeq10^{11}\,M_\odot$ is
$\left<f_{\rm esc}^{\rm 3D}\right>\simeq 10\,\%$. Roughly half of the LyC
photons are absorbed within scales of 100 pc, and the other half is absorbed in
the ISM ($\lesssim 2\, {\rm kpc}$). When the metallicity of the gas is
increased to $Z=0.02$, the escape fraction is significantly reduced to
$\left<f_{\rm esc}^{\rm 3D}\right>\simeq1\%$ because young stars are enshrouded
by their birth clouds for a longer time. In contrast, increasing the gas mass
by a factor of 5 leads to $\left<f_{\rm esc}^{\rm 3D}\right>\simeq 5\, \%$
because LyC photons are only moderately absorbed by the thicker disc. Our
experiments suggest that high metallicity is likely more responsible for the
low escape fractions observed in massive star-forming galaxies, supporting the
scenario in which the escape fraction is decreasing with increasing halo mass.
Finally, negligible correlation is observed between the escape fraction and
surface density of star formation or galactic outflow rates.
