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 H α 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 1D 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 Mpc^-1 < k < 2 Mpc^-1 in the 116-124, 146-154, and 186-194 MHz frequency bands. We further analyse the 2D 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 per cent on scales of k ∼ 1.2 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.