Results from EDGES High-Band: I. Constraints on Phenomenological Models for the Global $21$ cm Signal

TL;DR

This study uses EDGES High-Band observations to place constraints on models of the global 21 cm hydrogen signal during reionization and the First Light era, ruling out many phenomenological scenarios based on spectral data.

Contribution

First constraints on phenomenological models of the global 21 cm signal from EDGES High-Band data, including ruling out specific reionization durations and absorption feature models.

Findings

Ruling out reionization models with duration Δz ≤ 1 at z≈8.5.

Rejecting cold IGM scenarios with Δz ≤ 2.

Disfavoring broad Gaussian absorption features with Δz ≤ 4.

Abstract

We report constraints on the global $21$ cm signal due to neutral hydrogen at redshifts $14.8 \geq z \geq 6.5$. We derive our constraints from low foreground observations of the average sky brightness spectrum conducted with the EDGES High-Band instrument between September $7$ and October $26$, $2015$. Observations were calibrated by accounting for the effects of antenna beam chromaticity, antenna and ground losses, signal reflections, and receiver parameters. We evaluate the consistency between the spectrum and phenomenological models for the global $21$ cm signal. For tanh-based representations of the ionization history during the epoch of reionization, we rule out, at $\geq2\sigma$ significance, models with duration of up to $\Delta z = 1$ at $z\approx8.5$ and higher than $\Delta z = 0.4$ across most of the observed redshift range under the usual assumption that the $21$ cm spin temperature is much larger than the temperature of the cosmic microwave background (CMB) during reionization. We also investigate a `cold' IGM scenario that assumes perfect Ly$\alpha$ coupling of the $21$ cm spin temperature to the temperature of the intergalactic medium (IGM), but that the IGM is not heated by early stars or stellar remants. Under this assumption, we reject tanh-based reionization models of duration $\Delta z \lesssim 2$ over most of the observed redshift range. Finally, we explore and reject a broad range of Gaussian models for the $21$ cm absorption feature expected in the First Light era. As an example, we reject $100$ mK Gaussians with duration (full width at half maximum) $\Delta z \leq 4$ over the range $14.2\geq z\geq 6.5$ at $\geq2\sigma$ significance.