Partially Constrained Internal Linear Combination: a method for low-noise CMB foreground mitigation

TL;DR

The paper introduces the partially constrained ILC (pcILC) method, which optimizes the balance between foreground bias and variance in CMB data cleaning, leading to lower noise and residuals in simulated experiments.

Contribution

The novel pcILC method allows adjustable foreground suppression while controlling noise increase, improving upon standard and constrained ILC techniques.

Findings

Reduces variance by at least 50% for tSZ cleaning at high multipoles.

Effectively balances foreground nulling and noise increase.

Demonstrates improved noise reduction in CMB lensing reconstruction.

Abstract

Internal Linear Combination (ILC) methods are some of the most widely used multi-frequency cleaning techniques employed in CMB data analysis. These methods reduce foregrounds by minimizing the total variance in the coadded map (subject to a signal-preservation constraint), although often significant foreground residuals or biases remain. A modification to the ILC method is the constrained ILC (cILC), which explicitly nulls certain foreground components; however, this foreground nulling often comes at a high price for ground-based CMB datasets, with the map noise increasing significantly on small scales. In this paper we explore a new method, the partially constrained ILC (pcILC), which allows us to optimize the tradeoff between foreground bias and variance in ILC methods. In particular, this method allows us to minimize the variance subject to an inequality constraint requiring that the constrained foregrounds are reduced by at least a fixed factor, which can be chosen based on the foreground sensitivity of the intended application. We test our method on simulated sky maps for a Simons Observatory-like experiment; we find that for cleaning thermal Sunyaev-Zel'dovich (tSZ) contamination at $\ell \in [3000,4800]$, if a small tSZ residual of 20% of the standard ILC residual can be tolerated, the variance of the CMB temperature map is reduced by at least 50% over the cILC value. We also demonstrate an application of this method to reduce noise in CMB lensing reconstruction.