A hybrid map-$C_\ell$ component separation method for primordial CMB $B$-mode searches

TL;DR

This paper introduces a hybrid component separation method for multi-frequency CMB data that effectively reduces Galactic foreground contamination to enable primordial B-mode detection, with manageable increases in uncertainty.

Contribution

The paper presents a novel hybrid map-$C_ ext{ell}$ separation technique that combines advantages of existing methods, improving foreground removal in realistic observational conditions.

Findings

Reduces foreground contamination below $r_{FG} \\lesssim 5 \\times 10^{-4}$ for various models.

Increases statistical uncertainties by only 20-30%.

Applicable to large sky areas and different B-mode features.

Abstract

The observation of the polarised emission from the Cosmic Microwave Background (CMB) from future ground-based and satellite-borne experiments holds the promise of indirectly detecting the elusive signal from primordial tensor fluctuations in the form of large-scale $B$-mode polarisation. Doing so, however, requires an accurate and robust separation of the signal from polarised Galactic foregrounds. We present a component separation method for multi-frequency CMB observations that combines some of the advantages of map-based and power-spectrum-based techniques, and which is direcly applicable to data in the presence of realistic foregrounds and instrumental noise. We demonstrate that the method is able to reduce the contamination from Galactic foregrounds below an equivalent tensor-to-scalar ratio $r_{\rm FG}\lesssim5\times10^{-4}$, as required for next-generation observatories, for a wide range of foreground models with varying degrees of complexity. This bias reduction is associated with a mild $\sim20-30\%$ increase in the final statistical uncertainties, and holds for large sky areas, and for experiments targeting both the reionisation and recombination bumps in the $B$-mode power spectrum.