Improving cosmological analyses of HI clustering by reducing stochastic noise

TL;DR

This paper demonstrates that using optimal analysis methods can significantly improve cosmological parameter constraints from 21 cm intensity mapping by reducing stochastic noise effects, with potential improvements up to 50%.

Contribution

It provides a conservative estimate of the benefits of optimal analysis strategies over traditional power spectrum methods in 21 cm surveys, and offers measurements of HI bias and stochasticity.

Findings

Optimal analysis can improve parameter constraints by up to 50%.

Bias and stochasticity parameters for neutral hydrogen are measured and modeled.

Results highlight the importance of advanced analysis techniques for future 21 cm surveys.

Abstract

High-number-density tracers of large-scale structure, such as the HI-rich galaxies measured by 21 cm intensity mapping, have low sampling noise, making them particularly promising as cosmological probes. At large scales, this sampling noise can be subdominant to other scale-independent contributions to the power spectrum; such contributions arise from nonlinear bias, and exceed the sampling noise if at least one of the associated bias coefficients is sufficiently large. This has important consequences for cosmological constraints obtained from such tracers, since it indicates that using the power spectrum does not lead to optimal constraints even in the linear regime. In this paper, we provide a conservative estimate of the possible improvement in constraining power of a 21cm survey if one were to use an optimal analysis strategy (such as field-level analysis), where only the true sampling noise enters the error budget. We find that improvements in uncertainties on some cosmological parameters can be as large as 50%, depending on redshift, foreground cleaning efficiency, scales used in the analysis, and instrumental noise. One byproduct of our work is measurements of bias parameters and stochasticity for neutral hydrogen in the IllustrisTNG simulation over a wide range of redshifts; we provide simple fitting formulas for these measurements. Our results motivate further exploration of new optimal analysis techniques and provide important insights into the constraining power of current and future 21 cm surveys.