Decontamination of cosmological 21-cm maps

Abstract

We present a method for extracting the expected cosmological 21-cm signal from the epoch of re-ionization, taking into account contaminating radiations and random instrumental noise. The method is based on the maximum a posteriori probability (MAP) formalism and employs the coherence of the contaminating radiation along the line-of-sight and the three-dimensional correlations of the cosmological signal. We test the method using a detailed and comprehensive modelling of the cosmological 21-cm signal and the contaminating radiation. The signal is obtained using a high-resolution N-body simulation where the gas is assumed to trace the dark matter and is re-ionized by stellar radiation computed from semi-analytic galaxy formation recipes. We model contaminations to the cosmological signal from synchrotron and free–free galactic foregrounds and extragalactic sources including active galactic nuclei, radio haloes and relics, synchrotron and free–free emission from star-forming galaxies and free–free emission from dark matter haloes and the intergalactic medium. We provide tests of the reconstruction method for several rms values of instrumental noise from σN= 1 to 250 mK. For low instrumental noise, the recovered signal, along individual lines-of-sight, fits the true cosmological signal with a mean rms difference of drms≈ 1.7 ± 0.6 for σN= 1 mK, and drms≈ 4.2 ± 0.4 for σN= 5 mK. The one-dimensional power spectrum is nicely reconstructed for all values of σN considered here, while the reconstruction of the two-dimensional power spectrum and the Minkowski functionals is good only for noise levels of the order of few mK.