Bayesian constraints on the global 21-cm signal from the Cosmic Dawn

TL;DR

This paper introduces a Bayesian method, extsc{hibayes}, for extracting the faint global 21-cm signal from the Cosmic Dawn epoch amidst bright foregrounds, demonstrating its effectiveness on real observational data.

Contribution

The paper presents a novel Bayesian framework for isolating the global 21-cm signal, capable of handling complex foregrounds and applied to actual observational data from the EDGES experiment.

Findings

Successfully constrained the 21-cm signal amplitude to between -890 and 0 mK.

Determined the signal width to be greater than 6.5 MHz, indicating a broad feature.

Demonstrated the method's robustness with only 19 minutes of observational data.

Abstract

The birth of the first luminous sources and the ensuing epoch of reionization are best studied via the redshifted 21-cm emission line, the signature of the first two imprinting the last. In this work we present a fully-Bayesian method, \textsc{hibayes}, for extracting the faint, global (sky-averaged) 21-cm signal from the much brighter foreground emission. We show that a simplified (but plausible), Gaussian model of the 21-cm emission from the Cosmic Dawn epoch ($15 \lesssim z \lesssim 30$), parameterized by an amplitude $A_{\rm HI}$, a frequency peak $\nu_{\rm HI}$ and a width $\sigma_{\rm HI}$, can be extracted even in the presence of a structured foreground frequency spectrum (parameterized as a $7^{\rm th}$-order polynomial), provided sufficient signal-to-noise (400~hours of observation with a single dipole). We apply our method to an early, 19-minute long observation from the Large aperture Experiment to detect the Dark Ages, constraining the 21-cm signal amplitude and width to be $-890 < A_{\rm HI} < 0$ mK and $\sigma_{\rm HI} > 6.5$ MHz (corresponding to $\Delta z > 1.9$ at redshift $z \simeq 20$) respectively at the 95-per-cent confidence level in the range $13.2 < z < 27.4$ ($100 > \nu > 50$ MHz).