Improving CMB non-Gaussianity estimators using tracers of local structure

TL;DR

This paper proposes improved estimators for local non-Gaussianity in the CMB by using tracers like galaxy counts and lensing potential to remove ISW contributions, enhancing sensitivity.

Contribution

It introduces a nearly-optimal estimator for NL that leverages tracers of the ISW effect to reduce variance in non-Gaussianity measurements.

Findings

Variance can be reduced by up to 20% with galaxy counts at Planck sensitivity.

CMB lensing is an effective ISW tracer for future observations.

Good polarization data limits the fractional variance decrease.

Abstract

Local non-Gaussianity causes correlations between large scale perturbation modes and the small scale power. The large-scale CMB signal has contributions from the integrated Sachs Wolfe (ISW) effect, which does not correlate with the small scale power. If this ISW contribution can be removed, the sensitivity to local non-Gaussianity is improved. Gravitational lensing and galaxy counts can be used to trace the ISW contribution; in particular we show that the CMB lensing potential is highly correlated with the ISW signal. We construct a nearly-optimal estimator for the local non-Gaussianity parameter $\fnl$ and investigate to what extent we can use this to decrease the variance on ${\fnl}$. We show that the variance can be decreased by up to $20\%$ at Planck sensitivity using galaxy counts. CMB lensing is a good bias-independent ISW tracer for future more sensitive observations, though the fractional decrease in variance is small if good polarization data is also available.