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

Abstract

We report constraints on the global 21 cm signal due to neutral hydrogen at redshifts 14.8 ≥ z ≥ 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 ≥ 2σ significance, models with duration of up to ∆z = 1 at z ≈ 8.5 and higher than ∆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 Lyα 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 ∆z 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) ∆z ≤ 4 over the range 14.2 ≥ z ≥ 6.5 at ≥ 2σ significance.

Current and future observations of the high-z IGM with the 21cm line

“…Similar conclusions are drawn from observations of the global 21-cm signal in the 6 < z < 12 range that set joint constraints on the duration and mid-point of reionization (when x HI = 0.5) together with a T b > −100 mK (2σ ) upper limit amplitude on the 21-cm signal (Monsalve et al 2017; Figure 1). For a maximally cold model, this constraint implies T k (z * ) 5 K, again implying that reionization cannot be completely cold and the IGM must have been previously heated.…”

“…This value can be compared against the gas temperature expected in a "maximally cold" model, e.g. a model where the gas was never heated and the spin temperature was instantaneously coupled to the gas temperature when the first stars were formed (e.g., Bernardi et al 2016; Monsalve et al 2017). Although such model is somewhat unrealistic, it provides a lower bound to the gas temperature.…”

H I 21‐cm Cosmology and the Bispectrum: Closure Diagnostics in Massively Redundant Interferometric Arrays

“…Hence, the expected H I 21-cm signal from the epoch of reionization will be observed at frequencies from 100 MHz to 200 MHz. Most recently, there is a possible detection of the all-sky HI 21-cm signal from the Intergalactic Medium (IGM) at z ∼ 18, during the so-called "cosmic dawn," when the very first generation of stars form (Barkana, 2018;Bowman et al, 2018) Numerous low-frequency interferometers and total power experiments have been built with the thermal sensitivity adequate to detect the H I 21-cm signal from the reionization epoch (Beardsley et al, 2013;Monsalve et al, 2017;Thyagarajan et al, 2013). However, the task is complicated by the much stronger foreground continuum emission: the required dynamic range is about 50 dB in the mean brightness (Monsalve et al, 2017) and 40 dB for spatial fluctuations on the scales relevant to current interferometric observations .…”

Probing small scale primordial power spectrum with 21cm line global signal

“…In fact, models with the absorption trough peaked at z < 15 can also be severely constrained by the EDGES high-band result [19] in which the rms of δT b is 17 mK at 6.5 < z < 15 and 6 mK at 6.5 < z < 12 after polynomial foreground removal. Thus, we also use the EDGES high-band result to derive the constraint in this paper.…”

“…primordial fluctuations from the feature of the absorption trough of the global signal of 21cm line, particularly having the result from EDGES in mind. We utilize not only lowband data [10], but also high-band one [19] from which we can obtain constraints on the amplitude of small scale fluctuations, but also clearly see what scales (wave number) mainly affect the 21cm signal. Such an analysis would also bring us great insight towards future observations of 21cm line.…”