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Overcoming the Challenges of 21Cm Cosmology

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Overcoming the Challenges of 21Cm Cosmology Overcoming the Challenges of 21cm Cosmology By Jonathan Pober A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Astrophysics in the Graduate Division of the University of California, Berkeley Committee in charge: Professor Aaron Parsons, Chair Professor Carl Heiles Professor Adrian Lee Spring 2013 Overcoming the Challenges of 21cm Cosmology Copyright 2013 by Jonathan Pober 1 Abstract Overcoming the Challenges of 21cm Cosmology by Jonathan Pober Doctor of Philosophy in Astrophysics University of California, Berkeley Professor Aaron Parsons, Chair The highly-redshifted 21cm line of neutral hydrogen is one of the most promising and unique probes of cosmology for the next decade and beyond. The past few years have seen a number of dedicated experiments targeting the 21cm signal from the Epoch of Reionization (EoR) begin operation, including the LOw-Frequency ARray (LOFAR), the Murchison Widefield Array (MWA), and the Donald C. Backer Precision Array for Probing the Epoch of Reionization (PAPER). For these experiments to yield cosmological results, they require new calibration and analysis algorithms which will need to achieve unprecedented levels of separation between the 21cm signal and contaminating foreground emission. Although much work has been spent developing these algorithms over the past decade, their success or failure will ultimately depend on their ability to overcome the complications associated with real-world systems and their inherent complications. The work in this dissertation is closely tied to the late-stage commissioning and early observations with PAPER. The first two chapters focus on developing calibration algorithms to overcome unique problems arising in the PAPER system. To test these algorithms, I rely on not only simulations, but on commissioning observations, ultimately tying the success of the algorithm to its performance on actual, celestial data. The first algorithm works to correct gain-drifts in the PAPER system caused by the heating and cooling of various components (the amplifiers and above ground co-axial cables, in particular). It is shown that a simple measurement of the ambient temperature can remove ∼ 10% gain fluctuations in the observed brightness of calibrator sources. This result is highly encouraging for the ability of PAPER to remove a potentially dominant systematic in its power spectrum and cataloging measurements without resorting to a complicated system overhaul. The second new algorithm developed in this dissertation solves a major calibration challenge not just for PAPER, but for nearly all of a large class of new wide-field, drift- scanning radio telescopes: primary beam calibration in the presence of a poorly measured sky. Since these telescopes lack the ability to steer their primary beams, while seeing nearly the entire sky at once, a large number of calibrator sources are necessary to probe the entire beam 2 response. However, the catalogs of radio sources at low-frequencies are not reliable enough to achieve the level of primary beam accuraccy needed for 21cm cosmology experiments. I develop, test, and apply a new technique which — using only the assumption of symmetry around a 180◦ rotation — simultaneously solves for the primary beam and the flux density of large number of sources. In this dissertation, I also present the analysis of new observations from PAPER to test theoretical models which predict foreground emission is confined to a “wedge”-like region of cosmological Fourier space, leaving an “EoR window” free from contamination. For the first time in actual observations, these predictions are spectacularly confirmed. In many ways, this result shifts the burden for upcoming PAPER analysis from foreground removal to increased sensitivity. And although increasing sensitivity is no small feat in-and-of-itself, this result is highly encouraging for 21cm studies, as foreground removal was long-viewed as the principal challenge for this field. The final result in this dissertation is the application of the all the lessons learned building PAPER and the MWA to design a new experiment for 21cm studies at z ∼ 1 with the goal of measuring baryon acoustic oscillations (BAO). The design of the BAO Broadband and Broad-beam (BAOBAB) Array is described, and cosmological forecasts are presented. The bottom line is highly encouraging, suggesting that z ∼ 1 21cm observations can detect the neutral hydrogen power spectrum with a very modest (16 − 32 element) array, and that still reasonably sized (128 − 256 elements) arrays can produce significant advances in our knowledge of dark energy. i Contents Acknowledgments iv 1 Introduction1 1.1 Cosmic History..................................1 1.1.1 The Early Universe............................2 1.1.2 The Dark Ages..............................2 1.1.3 The Epoch of Reionization........................3 1.1.4 The Post-Reionization Epoch......................6 1.2 21cm Cosmology.................................7 1.2.1 21cm Emission at High Redshift.....................7 1.2.2 The Global Signal.............................8 1.2.3 The Spatially Fluctuating Signal....................9 1.3 Challenges of Observing the 21cm Signal.................... 11 1.3.1 The Faintness of the 21cm Signal.................... 11 1.3.2 The Brightness of Foreground Emission................. 13 1.3.3 Calibration and Foreground Removal.................. 13 1.4 21cm Experiments................................ 14 1.5 The Precision Array for Probing the Epoch of Reionization.......... 16 1.5.1 The PAPER Approach.......................... 16 1.6 Outline of This Dissertation........................... 17 2 Temperature Dependent Gains in the PAPER System 18 2.1 Introduction.................................... 18 2.2 Laboratory Measurements............................ 19 2.3 Analysis...................................... 19 2.4 Significance.................................... 22 2.5 Discussion..................................... 24 2.6 Implementation in South Africa......................... 24 3 A Technique for Primary Beam Calibration of Drift-Scanning, Wide-Field Antenna Elements 25 3.1 Introduction.................................... 25 Contents ii 3.2 Motivation..................................... 27 3.3 Methods...................................... 27 3.3.1 Obtaining Perceived Source Flux Densities............... 29 3.3.2 Gridding the Measurements....................... 30 3.3.3 Forming a Least-Squares Problem.................... 30 3.3.4 Using Deconvolution to Fill in Gaps in the Beam Model....... 34 3.3.5 Introduction of Prior Knowledge..................... 34 3.4 Application to Simulated Data.......................... 34 3.4.1 Simulations of Perceived Flux Density Tracks............. 35 3.4.2 Simulations of Visibilities........................ 35 3.5 Observed Data.................................. 38 3.5.1 Data Reduction.............................. 39 3.5.2 Results................................... 39 3.5.3 Tests of Validity.............................. 43 3.6 Conclusions.................................... 43 4 Opening the 21cm EoR Window: Measurements of Foreground Isolation with PAPER 46 4.1 Introduction.................................... 46 4.2 The Data..................................... 47 4.3 Analysis Techniques............................... 48 4.3.1 Delay Space CLEAN........................... 50 4.3.2 Power Spectra............................... 50 4.4 Results....................................... 51 4.5 Conclusions.................................... 55 5 The Baryon Acoustic Oscillation Broadband and Broad-beam Array: De- sign Overview and Sensitivity Forecasts 57 5.1 Introduction.................................... 58 5.2 The BAO Broadband and Broad-beam Array................. 58 5.2.1 Siting................................... 59 5.2.2 Analog System.............................. 60 5.2.3 Digital System.............................. 62 5.2.4 Configuration............................... 64 5.3 Predicted Cosmological Constraints from BAOBAB.............. 65 5.3.1 The 21cm Power Spectrum........................ 65 5.3.2 Sensitivity of an Array to the 21cm Signal............... 67 5.3.3 The Delay Spectrum Technique at z ∼ 1 ................ 74 5.3.4 Detecting the HI Power Spectrum.................... 76 5.3.5 Detecting Baryon Acoustic Oscillations................. 78 5.4 Discussion..................................... 84 5.4.1 Potential Shortcomings in the Analysis................. 84 Contents iii 5.4.2 Improving The Constraints........................ 86 5.5 Conclusions.................................... 87 6 Conclusions 89 6.1 Summary..................................... 89 6.2 Future Directions................................. 90 Bibliography 92 iv Acknowledgments So much work has been done the past few years, and so many people have supported me — it’s an impossible task to summarize it all in a few acknowledgments (especially when you’re working on a deadline). My sincerest apologies to anyone left out. The PAPER project on which so much of this dissertation is based is supported through the NSF-AST program (awards 0804508, 0901961, 1129258, and 1125558), the Mt. Cuba Astronomical Association, and by significant efforts by staff at NRAO’s Green Bank and Charlottesville sites. I also would like to thank our collaborators at SKA-SA for ensuring the smooth running of PAPER, and for minimizing the number of (long) trips I’ve had to make to South Africa. This research would not have happened without
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