Density reconstruction from biased tracers and its application to primordial non-Gaussianity

TL;DR

This paper develops a quadratic estimator formalism to reconstruct large-scale cosmic density modes from small-scale tracer correlations, improving constraints on primordial non-Gaussianity by mitigating cosmic variance in galaxy surveys.

Contribution

It introduces a novel reconstruction method incorporating nonlinear effects and demonstrates its effectiveness in enhancing primordial non-Gaussianity constraints in upcoming surveys.

Findings

Reconstructed modes tighten constraints on local primordial non-Gaussianity.

Estimator performs well on N-body simulations, matching theoretical expectations.

Reconstruction enables cosmic variance cancellation, similar to multitracer techniques.

Abstract

Large-scale Fourier modes of the cosmic density field are of great value for learning about cosmology because of their well-understood relationship to fluctuations in the early universe. However, cosmic variance generally limits the statistical precision that can be achieved when constraining model parameters using these modes as measured in galaxy surveys, and moreover, these modes are sometimes inaccessible due to observational systematics or foregrounds. For some applications, both limitations can be circumvented by reconstructing large-scale modes using the correlations they induce between smaller-scale modes of an observed tracer (such as galaxy positions). In this paper, we further develop a formalism for this reconstruction, using a quadratic estimator similar to the one used for lensing of the cosmic microwave background. We incorporate nonlinearities from gravity, nonlinear biasing, and local-type primordial non-Gaussianity, and verify that the estimator gives the expected results when applied to N-body simulations. We then carry out forecasts for several upcoming surveys, demonstrating that, when reconstructed modes are included alongside directly-observed tracer density modes, constraints on local primordial non-Gaussianity are generically tightened by tens of percents compared to standard single-tracer analyses. In certain cases, these improvements arise from cosmic variance cancellation, with reconstructed modes taking the place of modes of a separate tracer, thus enabling an effective "multitracer" approach with single-tracer observations.