Extracting bias using the cross-bispectrum: An EoR and 21 cm-[CII]-[CII] case study

TL;DR

This paper presents a novel method using the 21 cm-[CII]-[CII] cross-bispectrum to extract the bias factor during the Epoch of Reionization, offering robustness against foreground contamination and broad applicability.

Contribution

It introduces a bispectrum-based technique to measure the 21 cm bias factor that is less sensitive to foregrounds and validated with simulations, applicable across cosmic history.

Findings

The method accurately recovers <T_21> b_21(z) within 10% on large scales.

The cross-bispectrum is less affected by foreground contamination than auto-spectrum.

Simulations confirm the perturbative estimate's validity for <x_i> <~ 0.8.

Abstract

The amplitude of redshifted 21 cm fluctuations during the Epoch of Reionization (EoR) is expected to show a distinctive "rise and fall" behavior with decreasing redshift as reionization proceeds. On large scales (k <~ 0.1 Mpc^{-1}) this can mostly be characterized by evolution in the product of the mean 21 cm brightness temperature and a bias factor, b_21(z). This quantity evolves in a distinctive way that can help in determining the average ionization history of the intergalactic medium (IGM) from upcoming 21 cm fluctuation data sets. Here we consider extracting <T_21> b_21(z) using a combination of future redshifted 21 cm and [CII] line-intensity mapping data sets. Our method exploits the dependence of the 21 cm-[CII]-[CII] cross-bispectrum on the shape of triangle configurations in Fourier space. This allows one to determine <T_21> b_21(z) yet, importantly, is less sensitive to foreground contamination than the 21 cm auto-spectrum, and so can provide a valuable cross-check. We compare the results of simulated bispectra with second-order perturbation theory: on large scales the perturbative estimate of <T_21> b_21(z) matches the true value to within 10% for <x_i> <~ 0.8. We consider the 21 cm auto-bispectrum and show that this statistic may also be used to extract the 21 cm bias factor. Finally, we discuss the survey requirements for measuring the cross-bispectrum. Although we focus on the 21 cm-[CII]-[CII] bispectrum during reionization, our method may be of broader interest and can be applied to any two fields throughout cosmic history.