Separating polarized cosmological and galactic emissions for cosmic microwave background B-mode polarization experiments

The detection and characterization of the B mode of cosmic microwave background (CMB) polarization anisotropies will not be possible without a high-precision removal of the foreground contamination present in the microwave band. In this work, we study the relevance of the component-separation technique based on the Independent Component Analysis (ICA) for this purpose and investigate its performance in the context of a limited sky coverage observation and from the viewpoint of our ability to differentiate between cosmological models with different primordial B-mode content. We focus on the low Galactic emission sky patch centred at 40 degrees in right ascension and -45 in declination, corresponding to the target of several operating and planned CMB experiments and which, in many respects, adequately represents a typical 'clean' high-latitude sky. We consider two fiducial observations, one operating at low (40, 90 GHz) frequencies and one at high (150, 350 GHz) frequencies and thus dominated by the synchrotron and thermal dust emission, respectively. We use foreground templates simulated in accordance with the existing observations in the radio and infrared bands, as well as the Wilkinson Microwave Anisotropy Probe (WMAP) and Archeops data and model the CMB emission adopting the current best-fitting cosmological model, with an amplitude of primordial gravitational waves set to either zero or 10 per cent. We use a parallel version of the FASTICA code to explore a substantial parameter space including Gaussian pixel noise level, observed sky area and the amplitude of the foreground emission and employ large Monte Carlo simulations to quantify errors and biases pertinent to the reconstruction for different choices of the parameter values. We identify a large subspace of the parameter space for which the quality of the CMB reconstruction is excellent, i.e. where the errors and biases introduced by the separation are found to be comparable or lower than the uncertainty due to the cosmic variance and instrumental noise. For both the cosmological models, with and without the primordial gravitational waves, we find that FASTICA performs extremely well even in the cases when the B-mode CMB signal is up to a few times weaker than the foreground contamination and the noise amplitude is comparable with the total CMB polarized emission. In addition, we discuss limiting cases of the noise and foreground amplitudes, for which the ICA approach fails. Although our conclusions are limited by the absence of systematics in the simulated data, these results indicate that these component-separation techniques could play a crucial role in the forthcoming experiments aiming at the detection of B modes in the CMB polarization.