The 21‐cm emission from the reionization epoch: extended and point source foregrounds

Fluctuations in the redshifted 21‐cm emission from neutral hydrogen probe the epoch of reionization. We examine the observability of this signal and the impact of extragalactic foreground radio sources (both extended and point‐like). We use cosmological simulations to predict the angular correlation functions of intensity fluctuations due to unresolved radio galaxies, cluster radio haloes and relics and free–free emission from the interstellar and intergalactic medium at the frequencies and angular scales relevant for the proposed 21‐cm tomography. In accord with previous findings, the brightness temperature fluctuations due to foreground sources are much larger than those from the primary 21‐cm signal at all scales. In particular, diffuse cluster radio emission, which has been previously neglected, provides the most significant foreground contamination. However, we show that the contribution to the angular fluctuations at scales θ≳ 1 arcmin is dominated by the spatial clustering of bright foreground sources. This excess can be removed if sources above flux levels S ≳ 0.1 mJy (out to redshifts of z ∼ 1 and z ∼ 2 for diffuse and point sources, respectively) are detected and removed. Hence, efficient source removal may be sufficient to allow the detection of angular fluctuations in the 21‐cm emission free of extragalactic foregrounds at θ≳ 1 arcmin. In addition, the removal of sources above S = 0.1 mJy also reduces the foreground fluctuations to roughly the same level as the 21‐cm signal at scales θ≲ 1 arcmin. This should allow the substraction of the foreground components in frequency space, making it possible to observe in detail the topology and history of reionization.