Combining ILC and moment expansion techniques for extracting average-sky signals and CMB anisotropies

TL;DR

This paper introduces a novel constrained ILC method combining moment expansion techniques to improve extraction of CMB spectral distortions and anisotropies, demonstrating enhanced accuracy in foreground removal and signal recovery.

Contribution

The paper develops exttt{MILC}, a heavily constrained ILC approach integrating physics-informed constraints and moment expansion, enabling precise measurement of tiny spectral distortions and improved CMB anisotropy analysis.

Findings

Successfully recovered monopolar spectral distortion signals.

Reduced foreground biases in CMB anisotropy measurements.

Demonstrated potential for 21cm and B-mode analysis.

Abstract

The method of weighted addition of multi-frequency maps, more commonly referred to as {\it Internal Linear Combination} (ILC), has been extensively employed in the measurement of cosmic microwave background (CMB) anisotropies and its secondaries along with similar application in 21cm data analysis. Here we argue and demonstrate that ILC methods can also be applied to data from absolutely-calibrated CMB experiments to extract average-sky signals in addition to the conventional CMB anisotropies. The performance of the simple ILC method is, however, limited, but can be significantly improved by adding constraints informed by physics and existing empirical information. In recent work, a moment description has been introduced as a technique of carrying out high precision modeling of foregrounds in the presence of inevitable averaging effects. We combine these two approaches to construct a heavily constrained form of the ILC, dubbed \milc, which can be used to recover tiny monopolar spectral distortion signals in the presence of realistic foregrounds and instrumental noise. This is a first demonstration for measurements of the monopolar and anisotropic spectral distortion signals using ILC and extended moment methods. We also show that CMB anisotropy measurements can be improved, reducing foreground biases and signal uncertainties when using the \milc. While here we focus on CMB spectral distortions, the scope extends to the 21cm monopole signal and $B$-mode analysis. We briefly discuss augmentations that need further study to reach the full potential of the method.