Contribution of Galactic free–free emission to the foreground for EoR signal in SKA experiments

ABSTRACT 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 103.5–102.0, 103.0–101.3, and 102.5–101.0 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.