Effective bias expansion for circumventing 21 cm foregrounds

TL;DR

This paper proposes an effective bias expansion method combined with density information to indirectly recover the 21 cm signal from the Epoch of Reionization, mitigating foreground contamination.

Contribution

It introduces a systematic bias expansion approach using effective field theory to improve 21 cm signal recovery amidst foregrounds.

Findings

Achieves ~30% cross-correlation with true 21 cm field under optimistic assumptions.

Using high-redshift galaxy surveys increases cross-correlation to 50-70%.

Method can be combined with foreground mitigation to enhance signal detection.

Abstract

The 21 cm line of neutral hydrogen is a promising probe of the Epoch of Reionization (EoR) but suffers from contamination by spectrally smooth foregrounds, which obscure the large-scale signal along the line of sight. We explore the possibility of indirectly extracting the 21 cm signal in the presence of foregrounds by taking advantage of nonlinear couplings between large and small scales, both at the level of the density field and a biased tracer of that field. When combined with effective field theory techniques, bias expansions allow for a systematic treatment of nonlinear mode-couplings in the map between the underlying density field and the 21 cm field. We apply an effective bias expansion to density information generated with $\texttt{21cmFAST}$ using a variety of assumptions about which information can be recovered by different surveys and density estimation techniques. We find that the 21 cm signal produced by our effective bias expansion, combined with our least optimistic foreground assumptions, produces $\sim30\%$ cross-correlations with the true 21 cm field. Providing complementary density information from high-redshift galaxy surveys yields a cross-correlation of 50-70%. The techniques presented here may be combined with foreground mitigation strategies in order to improve the recovery of the large-scale 21 cm signal.