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, 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 z 30), parameterized by an amplitude A HI , a frequency peak ν HI and a width σ HI , can be extracted even in the presence of a structured foreground frequency spectrum (parameterized as a 7 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 HI < 0 mK and σ HI > 6.5 MHz (corresponding to ∆z > 1.9 at redshift z 20) respectively at the 95-per-cent confidence level in the range 13.2 < z < 27.4 (100 > ν > 50 MHz).

Modeling the Radio Foreground for Detection of CMB Spectral Distortions from the Cosmic Dawn and the Epoch of Reionization

“…The reionization signals are of 10-100 mK brightness temperature, whereas the foregrounds are 100s to 10,000s of K; the spectral dynamic range required for the detection of the global EoR signature in any measurement of the absolute spectrum of the radio sky is about 10 3 -10 6 . It is therefore not surprising that considerable effort has been placed on the development of methods for the separation of the faint 21-cm signals from the substantially brighter foreground component in measurement data in the context of detection of both the global and statistical EoR signals (see, for example, Gnedin & Shaver 2004;Morales et al 2006;Vedantham et al 2012;Liu et al 2013;Thyagarajan et al 2013;Harker 2015;Bernardi et al 2016;Chapman et al 2016;Nhan et al 2017). …”

A Space-based Observational Strategy for Characterizing the First Stars and Galaxies Using the Redshifted 21 cm Global Spectrum

“…These models rely on, at most, five components to describe the spectral content of the foreground over several decades in frequency. Global measurements from the Experiment to Detect the Global EoR Signature (EDGES), Sonda Cosmológica de las Islas para la Detección de Hidrógeno Neutro (SCI-HI), Shaped Antenna measurement of the background RAdio Spectrum (SARAS), and Large-Aperture Experiment to Detect the Dark Ages (LEDA) provide further validation of the intrinsic foreground smoothness (Rogers & Bowman 2008;Voytek et al 2014;Patra et al 2015;Bernardi et al 2016;Mozdzen et al 2017).…”

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

“…Bernardi et al (2016) presented a -890 mK upper limit on the amplitude of the 21-cm absorption in the 13 < z < 27 range, implying T k 3 K. A maximally cold model, however, leads to T k,c (z = 25) ∼ 12 K, therefore more sensitive observations are needed to place meaningful constraints on the IGM temperature prior reionization.…”

“…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.…”