A Monte Carlo Implementation of Galactic Free-Free Emission for the EoR Foreground Models

Abstract: The overwhelming foreground causes severe contamination on the detection of 21-cm signal 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 observation remains unclear. To better constrain this emission, we perform the Monte Carlo simulation of H$\alpha$ emission, which comprises direct and scattered H$\alpha$ radiation from HII regions and warm ionized medium (WIM). The positions and radii of HII regions are quoted from the WISE HII catalog, and the WIM is described by an axisymmetric model. The scattering is off dust and free electrons that are realized by applying an exponential fitting to the HI4PI HI map and an exponential disk model, respectively. The simulated H$\alpha$ intensity, the Simfast21 software, and the latest SKA1-Low layout configuration are employed to simulate the SKA "observed" images of Galactic free-free emission and the EoR signal. By analyzing the one-dimensional power spectra, we find that the Galactic free-free emission can be about $10^{5.4}$-$10^{2.1}$, $10^{5.0}$-$10^{1.7}$, and $10^{4.3}$-$10^{1.1}$ 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 MHz frequency bands, respectively. We further calculate the two-dimensional power spectra inside the EoR window and show that the power leaked by Galactic free-free emission can still be significant, as the power ratios can reach about $110\%$-$8000\%$, $30\%$-$2400\%$, and $10\%$-$250\%$ on scales of $0.5~{\rm Mpc^{-1}} \lesssim k \lesssim 1~{\rm Mpc^{-1}}$ in three frequency bands. Therefore, we indicate that the Galactic free-free emission should be carefully treated in future EoR detections.