Measurements of one-point statistics in 21 cm intensity maps via foreground avoidance strategy

TL;DR

This study assesses the feasibility of measuring one-point statistics of high-redshift 21 cm fluctuations using a foreground avoidance strategy with HERA, proposing a rolling filter method to optimize measurements despite foreground contamination.

Contribution

It introduces a novel rolling wedge-cut filtering technique to improve 21 cm one-point statistic measurements in the presence of foregrounds during reionization.

Findings

The band center is least affected by wedge-cut bias.

The rolling filter method enables better reconstruction of 21 cm maps.

HERA can detect skewness and kurtosis evolution if foreground leakage is managed.

Abstract

Measurements of the one-point probability distribution function and higher-order moments (variance, skewness, and kurtosis) of the high-redshift 21 cm fluctuations are among the most direct statistical probes of the non-Gaussian nature of structure formation and evolution during reionization. However, contamination from astrophysical foregrounds and instrument systematics pose significant challenges in measuring these statistics in real observations. In this work, we use forward modelling to investigate the feasibility of measuring 21 cm one-point statistics through a foreground avoidance strategy. Leveraging the characteristic wedge-shape of the foregrounds in k-space, we apply a wedge-cut filter that removes the foreground contaminated modes from a mock data set based on the Hydrogen Epoch of Reionization Array (HERA) instrument, and measure the one-point statistics from the image-space representation of the remaining non-contaminated modes. We experiment with varying degrees of wedge-cutting over different frequency bandwidths and find that the centre of the band is the least susceptible to bias from wedge-cutting. Based on this finding, we introduce a rolling filter method that allows reconstruction of an optimal wedge-cut 21~cm intensity map over the full bandwidth using outputs from wedge-cutting over multiple sub-bands. We perform Monte Carlo simulations to show that HERA should be able to measure the rise in skewness and kurtosis near the end of reionization with the rolling wedge-cut method if foreground leakage from the Fourier transform window function can be controlled.