Foreground Removal vs. Foreground Avoidance: Contamination of the EoR Window

The removal of the Galactic and extragalactic foregrounds remains a major challenge for those wishing to make a detection of the Epoch of Reionization 21-cm signal. Multiple methods of modelling these foregrounds with varying levels of assumption have been trialled and shown promising recoveries on simulated data. Recently however there has been increased discussion of using the expected shape of the foregrounds in Fourier space to define an EoR window free of foreground contamination. By carrying out analysis within this window only, one can avoid the foregrounds and any statistical bias they might introduce. In this paper we discuss the advantages and disadvantages of both foreground removal and foreground avoidance. We create a series of simulations with noise levels in line with both current and future experiments and compare
the recovered statistical cosmological signal from both methods. We also relax the assumption that the foregrounds are smooth by introducing a Gaussian random factor along the line-of-sight and then also spatially. We find that, while foreground avoidance enables a slightly better recovery at high kperp, the most significant part of the cosmological signal is lost. While the EoR window is almost entirely obscured for a non-smooth foreground simulation, GMCA is able to recover the signal almost as well as for a smooth foreground for a line-of-sight variation.