Contribution of Galactic free-free emission to the foreground for EoR
  signal in SKA experiments
  The overwhelming foreground contamination hinders the accurate detection of
the 21-cm signal of neutral hydrogen during the Epoch of Reionization (EoR).
Among various foreground components, the Galactic free-free emission is less
studied, so that its impact on the EoR observations remains unclear. In this
work, we employ the observed $\rm H\alpha$ intensity map with the correction of
dust absorption and scattering, the Simfast21 software, and the latest SKA1-Low
layout configuration to simulate the SKA "observed" images of Galactic
free-free emission and the EoR signal. By calculating the one-dimensional power
spectra from the simulated image cubes, we find that the Galactic free-free
emission is about $10^{3.5}$-$10^{2.0}$, $10^{3.0}$-$10^{1.3}$, and
$10^{2.5}$-$10^{1.0}$ times more luminous than the EoR signal on scales of
$0.1~\rm Mpc^{-1} < k < 2~\rm Mpc^{-1}$ in the $116$-$124$, $146$-$154$, and
$186$-$194$ ${\rm MHz}$ frequency bands. We further analyse the two-dimensional
power spectra inside the properly defined EoR window and find that the leaked
Galactic free-free emission can still cause non-negligible contamination, as
the ratios of its power (amplitude squared) to the EoR signal power can reach
about $200\%$, $60\%$, and $15\%$ on scales of $1.2~\rm Mpc^{-1}$ in three
frequency bands, respectively. Therefore, we conclude that the Galactic
free-free emission, as a severe contaminating foreground component, needs to be
carefully treated in the forthcoming deep EoR observations.
