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

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 $A_{\mathrm{HI}}$, a frequency peak $\hat{I}{½}_{\mathrm{HI}}$ and a width $\ddot{I}{ƒ}_{\mathrm{HI}}$, can be extracted even in the presence of a structured foreground frequency spectrum (parameterized as a $7^{\mathrm{th}}$-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 $-890<A_{\mathrm{HI}}<0$ mK and $\ddot{I}{ƒ}_{\mathrm{HI}}>6.5$ MHz (corresponding to $\hat{I}”z>1.9$ at redshift $z\simeq 20$) respectively at the 95-per-cent confidence level in the range $13.2<z<27.4$ ($100>\hat{I}½>50$ MHz).

10 pages, 8 figures, MNRAS accepted