Adrian Liu

NEW LIMITS ON 21 cm EPOCH OF REIONIZATION FROM PAPER-32 CONSISTENT WITH AN X-RAY HEATED INTERGALACTIC MEDIUM AT z = 7.7

We present new constraints on the 21cm Epoch of Reionization (EoR) power spectrum derived from 3 months of observing with a 32-antenna, dual-polarization deployment of the Donald C. Backer Precision Array for Probing the Epoch of Reionization (PAPER) in South Africa. In this paper, we demonstrate the efficacy of the delay-spectrum approach to avoiding foregrounds, achieving over 8 orders of magnitude of foreground suppression (in mK). Combining this approach with a procedure for removing off-diagonal covariances arising from instrumental systematics, we achieve a best 2σ upper limit of (41mK) for k = 0.27 h Mpc−1 at z = 7.7. This limit falls within an order of magnitude of the brighter predictions of the expected 21cm EoR signal level. Using the upper limits set by these measurements, we generate new constraints on the brightness temperature of 21cm emission in neutral regions for various reionization models. We show that for several ionization scenarios, our measurements are inconsistent with cold reionization. That is, heating of the neutral intergalactic medium (IGM) is necessary to remain consistent with the constraints we report. Hence, we have suggestive evidence that by z = 7.7, the HI has been warmed from its cold primordial state, probably by X-rays from high-mass X-ray binaries or mini-quasars. The strength of this evidence depends on the ionization state of the IGM, which we are not yet able to constrain. This result is consistent with standard predictions for how reionization might have proceeded.We present new constraints on the 21 cm Epoch of Reionization (EoR) power spectrum derived from three months of observing with a 32 antenna, dual-polarization deployment of the Donald C. Backer Precision Array for Probing the Epoch of Reionization in South Africa. In this paper, we demonstrate the efficacy of the delay-spectrum approach to avoiding foregrounds, achieving over eight orders of magnitude of foreground suppression (in mK2). Combining this approach with a procedure for removing off-diagonal covariances arising from instrumental systematics, we achieve a best 2σ upper limit of (41 mK)2 for k = 0.27 h Mpc–1 at z = 7.7. This limit falls within an order of magnitude of the brighter predictions of the expected 21 cm EoR signal level. Using the upper limits set by these measurements, we generate new constraints on the brightness temperature of 21 cm emission in neutral regions for various reionization models. We show that for several ionization scenarios, our measurements are inconsistent with cold reionization. That is, heating of the neutral intergalactic medium (IGM) is necessary to remain consistent with the constraints we report. Hence, we have suggestive evidence that by z = 7.7, the H I has been warmed from its cold primordial state, probably by X-rays from high-mass X-ray binaries or miniquasars. The strength of this evidence depends on the ionization state of the IGM, which we are not yet able to constrain. This result is consistent with standard predictions for how reionization might have proceeded.

PAPER-64 Constraints on Reionization: The 21 cm Power Spectrum at z = 8.4

© 2015. The American Astronomical Society. All rights reserved. In this paper, we report new limits on 21 cm emission from cosmic reionization based on a 135 day observing campaign with a 64-element deployment of the Donald C. Backer Precision Array for Probing the Epoch of Reionization in South Africa. This work extends the work presented in Parsons et al. with more collecting area, a longer observing period, improved redundancy-based calibration, improved fringe-rate filtering, and updated power-spectral analysis using optimal quadratic estimators. The result is a new 2σ upper limit on Δ2(k) of (22.4 mK)2 in the range < k < 0.5h Mpc-1 at z = 8.4. This represents a three-fold improvement over the previous best upper limit. As we discuss in more depth in a forthcoming paper, this upper limit supports and extends previous evidence against extremely cold reionization scenarios. We conclude with a discussion of implications for future 21 cm reionization experiments, including the newly funded Hydrogen Epoch of Reionization Array.

An improved method for 21-cm foreground removal

21-cm tomography is expected to be difficult in part because of serious foreground contamination. Previous studies have found that line-of-sight approaches are capable of cleaning foregrounds to an acceptable level on large spatial scales, but not on small spatial scales. In this paper, we introduce a Fourier space formalism for describing the line-of-sight methods, and use it to introduce an improved new method for 21-cm foreground cleaning. Heuristically, this method involves fitting foregrounds in Fourier space using weighted polynomial fits, with each pixel weighted according to its information content. We show that the new method reproduces the old one on large angular scales, and gives marked improvements on small scales at essentially no extra computational cost.

A method for 21 cm power spectrum estimation in the presence of foregrounds

The technique of 21 cm tomography promises to be a powerful tool for estimating cosmological parameters, constraining the epoch of reionization, and probing the so-called dark ages. However, realizing this promise will require the extraction of a cosmological power spectrum from beneath overwhelmingly large sources of foreground contamination. In this paper, we develop a unified matrix-based framework for foreground subtraction and power spectrum estimation, which allows us to quantify the errors and biases that arise in the power spectrum as a result of foreground subtraction. We find that existing line-of-sight foreground subtraction proposals can lead to substantial mode mixing as well as residual noise and foreground biases, whereas our proposed inverse-variance foreground subtraction eliminates noise and foreground biases, gives smaller error bars, and produces less correlated measurements of the power spectrum. We also numerically confirm the intuitive belief in the literature that 21 cm foreground subtraction is best done using frequency rather than angular information.

Epoch of reionization window. I. Mathematical formalism

have neglected a rigorous examination of the error statistics associated with the wedge. Using a quadratic estimator formalism applied to the interferometric measurement equation, we provide a framework for such a rigorous analysis (incorporating a fully covariant treatment of errors). Additionally, we nd that there are strong error correlations at high spatial wavenumbers that have so far been neglected in sensitivity derivations. These error correlations substantially degrade the sensitivity of arrays relying on contributions from long baselines, compared to what one would estimate assuming uncorrelated errors.

Hydrogen Epoch of Reionization Array (HERA)

The Hydrogen Epoch of Reionization Array (HERA http://reionization.org) is a staged experiment that uses the unique properties of the 21-cm line from neutral hydrogen to probe the Epoch of Reionization (EOR). During this epoch, roughly 0.3-1 billion years after the Big Bang, the first galaxies and black holes heated and reionized the early Universe. Direct observation of the large scale structure of reionization and its evolution with time will have a profound impact on our understanding of the birth of the first galaxies and black holes, their influence on the intergalactic medium (IGM), and cosmology. This paper will provide an overview of the project and describe the design of the HERA receiving element.

Will point sources spoil 21-cm tomography?

21-cm tomography is emerging as a promising probe of the cosmological dark ages and the epoch of reionization, as well as a tool for observational cosmology in general. However, serious sources of foreground contamination must be subtracted for experimental efforts to be viable. In this paper, we focus on the removal of unresolved extragalactic point sources with smooth spectra, and evaluate how the residual foreground contamination after cleaning depends on instrumental and algorithmic parameters. A crucial but often ignored complication is that the synthesized beam of an interferometer array shrinks towards higher frequency, causing complicated frequency structure in each sky pixel as ‘frizz’ far from the beam centre contracts across unresolved radio sources. We find that current-generation experiments should none the less be able to clean out this point source contamination adequately, and quantify the instrumental and algorithmic design specifications required to meet this foreground challenge.

Multiredshift limits on the 21 cm power spectrum from paper

The epoch of the reionization (EoR) power spectrum is expected to evolve strongly with redshift, and it is this variation with cosmic history that will allow us to begin to place constraints on the physics of reionization. The primary obstacle to the measurement of the EoR power spectrum is bright foreground emission. We present an analysis of observations from the Donald C. Backer Precision Array for Probing the Epoch of Reionization (PAPER) telescope, which place new limits on the H i power spectrum over the redshift range of , extending previously published single-redshift results to cover the full range accessible to the instrument. To suppress foregrounds, we use filtering techniques that take advantage of the large instrumental bandwidth to isolate and suppress foreground leakage into the interesting regions of k-space. Our 500 hr integration is the longest such yet recorded and demonstrates this method to a dynamic range of 104. Power spectra at different points across the redshift range reveal the variable efficacy of the foreground isolation. Noise-limited measurements of Δ2 at k = 0.2 hr Mpc−1 and z = 7.55 reach as low as (48 mK)2 (1σ). We demonstrate that the size of the error bars in our power spectrum measurement as generated by a bootstrap method is consistent with the fluctuations due to thermal noise. Relative to this thermal noise, most spectra exhibit an excess of power at a few sigma. The likely sources of this excess include residual foreground leakage, particularly at the highest redshift, unflagged radio frequency interference, and calibration errors. We conclude by discussing data reduction improvements that promise to remove much of this excess.

How well can we measure and understand foregrounds with 21-cm experiments?

Before it becomes a sensitive probe of the epoch of reionization, the dark ages and fundamental physics, 21-cm tomography must successfully contend with the issue of foreground contamination. Broad-band foreground sources are expected to be roughly 4 orders of magnitude larger than any cosmological signals, so precise foreground models will be necessary. Such foreground models often contain a large number of parameters, reflecting the complicated physics that governs foreground sources. In this paper, we concentrate on spectral modelling (neglecting, for instance, bright point source removal from spatial maps) and show that 21-cm tomography experiments will likely not be able to measure these parameters without large degeneracies, simply because the foreground spectra are so featureless and generic. However, we show that this is also an advantage, because it means that the foregrounds can be characterized to a high degree of accuracy once a small number of parameters (likely three or four, depending on one’s instrumental specifications) are measured. This provides a simple understanding for why 21-cm foreground subtraction schemes are able to remove most of the contaminants by suppressing just a small handful of simple spectral forms. In addition, this suggests that the foreground modelling process should be relatively simple and will likely not be an impediment to the foreground subtraction schemes that are necessary for a successful 21-cm tomography experiment.

PAPER-64 CONSTRAINTS on REIONIZATION. II. the TEMPERATURE of the z = 8.4 INTERGALACTIC MEDIUM

© 2015. The American Astronomical Society. All rights reserved. We present constraints on both the kinetic temperature of the intergalactic medium (IGM) at z = 8.4, and on models for heating the IGM at high-redshift with X-ray emission from the first collapsed objects. These constraints are derived using a semi-analytic method to explore the new measurements of the 21 cm power spectrum from the Donald C. Backer Precision Array for Probing the Epoch of Reionization (PAPER), which were presented in a companion paper, Ali et al. Twenty-one cm power spectra with amplitudes of hundreds of mK2 can be generically produced if the kinetic temperature of the IGM is significantly below the temperature of the cosmic microwave background (CMB); as such, the new results from PAPER place lower limits on the IGM temperature at z = 8.4. Allowing for the unknown ionization state of the IGM, our measurements find the IGM temperature to be above ≈5 K for neutral fractions between 10% and 85%, above ≈7 K for neutral fractions between 15% and 80%, or above ≈10 K for neutral fractions between 30% and 70%. We also calculate the heating of the IGM that would be provided by the observed high redshift galaxy population, and find that for most models, these galaxies are sufficient to bring the IGM temperature above our lower limits. However, there are significant ranges of parameter space that could produce a signal ruled out by the PAPER measurements; models with a steep drop-off in the star formation rate density at high redshifts or with relatively low values for the X-ray to star formation rate efficiency of high redshift galaxies are generally disfavored. The PAPER measurements are consistent with (but do not constrain) a hydrogen spin temperature above the CMB temperature, a situation which we find to be generally predicted if galaxies fainter than the current detection limits of optical/NIR surveys are included in calculations of X-ray heating.