The Optical Depth of Foregrounds for the Highest Redshift 21 cm Signals

TL;DR

This study assesses the optical thickness of foreground emissions at low frequencies to determine regions where the 21 cm cosmological signal from the highest redshifts might be obscured, impacting future observations.

Contribution

It introduces a spectral modeling approach to map the sky's optical depth for high-redshift 21 cm signals using LWA1 and Haslam data.

Findings

Approximately 25% of the sky may be optically thick at the highest redshifts.

Current data limitations prevent definitive conclusions but suggest significant foreground opacity.

Evidence for sky regions obscuring 21 cm signals is at about 1 sigma significance.

Abstract

Foreground emission makes it difficult to detect the highly-redshifted cosmological 21 cm signal at any frequency. However, at low frequencies foregrounds are likely to become optically thick, which would make it completely impossible to see a 21 cm signal behind them. To find out which regions of the sky might be optically thick for the highest redshifts of the 21 cm signal, we fit the measurements from LWA1 and the Haslam 408 MHz map with a two-component spectral model and calculate the frequency-dependent foreground optical depth point-by-point across the sky. Limitations of the current data prevent us from making any strong conclusions at high statistical significance, but there is suggestive evidence ($\sim1\sigma$) that as much as 25% of the sky could be obscured for the highest redshift 21 cm signals.