Global 21 cm signal experiments: A designer’s guide

Abstract

The global (i.e. spatially averaged) spectrum of the redshifted 21 cm line has generated much experimental interest lately, thanks to its potential to be a direct probe of the Epoch of Reionization and the Dark Ages, during which the first luminous objects formed. Since the cosmological signal in question has a purely spectral signature, most experiments that have been built, designed, or proposed have essentially no angular sensitivity. This can be problematic because with only spectral information, the expected global 21 cm signal can be difficult to distinguish from foreground contaminants such as Galactic synchrotron radiation, since both are spectrally smooth and the latter is many orders of magnitude brighter. In this paper, we establish a systematic mathematical framework for global signal data analysis. The framework removes foregrounds in an optimal manner, complementing spectra with angular information. We use our formalism to explore various experimental design trade-offs, and find that 1) with spectral-only methods, it is mathematically impossible to mitigate errors that arise from uncertainties in one's foreground model; 2) foreground contamination can be significantly reduced for experiments with fine angular resolution; 3) most of the statistical significance in a positive detection during the Dark Ages comes from a characteristic high-redshift trough in the 21 cm brightness temperature; 4) Measurement errors decrease more rapidly with integration time for instruments with fine angular resolution; and 5) Better foreground models can help reduce errors, but once a modeling accuracy of a few percent is reached, significant improvements in accuracy will be required to further improve the measurements. We show that if observations and data analysis algorithms are optimized based on these findings, an instrument with a 5• wide beam can achieve highly significant detections (greater than 5σ) of even extended (high ∆z) reionization scenarios after integrating for 500 hrs. This is in strong contrast to instruments without angular resolution, which cannot detect gradual reionization. Ionization histories that are more abrupt can be detected with our fiducial instrument at the level of 10's to 100's of σ. The expected errors are similarly low during the Dark Ages, and can yield a 25σ detection of the expected cosmological signal after only 100 hrs of integration.PACS numbers: 95.75.Pq,98.80.Es

PROBING THE DARK AGES AT z ∼ 20: THE SCI-HI 21 cm ALL-SKY SPECTRUM EXPERIMENT

“…Expansion of the analysis to frequencies below 60 MHz will require consideration of the ionospheric ducting structure they describe. The residuals are also lower than those predicted by Liu et al (2013), particularly at lower frequencies. This implies that the the ∼10% variance of the GSM angular structure assumed in Liu et al (2013) contains significant finescale mapping errors such as striping due to reciever gain drift.…”

“…The magnitude and width of the trough provide valuable information on the properties of the first stars and X-ray sources. This trough is also a more easily measurable feature in the all-sky spectrum (Shaver et al 1999;Madau et al 1997;Tozzi et al 2000;Liu et al 2013) than the slope in the spectrum that occurs at lower redshifts as the universe is reionized.…”

“…INTRODUCTION Emission and absorption due to the 21 cm spin flip transition of neutral Hydrogen (HI) have emerged as valuable tools to probe the physics of the Universe across a set of cosmological eras ranging from the pre-star dark ages to the epoch of reionization to the era of acceleration (Loeb & Zaldarriaga 2004;Cooray 2004;Bharadwaj & Ali 2004;Carilli et al 2004;Furlanetto et al 2006;Pritchard & Loeb 2010Liu et al 2013). Experiments studying the universe through the 21 cm transition are of two kinds: mapping experiments that measure the power spectrum of 21 cm fluctations and all-sky absolute brightness experiments that measure the averaged 21 cm brightness temperature.…”

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). …”

CALIBRATION OF THE EDGES HIGH-BAND RECEIVER TO OBSERVE THE GLOBAL 21 cm SIGNATURE FROM THE EPOCH OF REIONIZATION

“…For reference, Figure 1 shows three phenomenological models for this signal, parameterized in terms of the redshift (z r ) and duration (Dz) of reionization (Bowman & Rogers 2010;Pritchard & Loeb 2010;Morandi & Barkana 2012;Liu et al 2013;Mirocha et al 2015;Harker et al 2016).…”