The 21-cm line of neutral hydrogen is a promising probe of the epoch of reionization 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 21 cmFAST 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 $\sim 30\%$ 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.

Close

PrevNext

View figure in article

• Cosmology
• Large scale structure of the Universe
• Radio, microwave, & sub-mm astronomy