Large-scale 21-cm Cosmology with LOFAR and AARTFAAC
Gehlot, Bharat Kumar
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Publication date: 2019
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Gehlot, B. K. (2019). Large-scale 21-cm Cosmology with LOFAR and AARTFAAC. University of Groningen.
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English summary
General Context
Understanding the formation and evolution of the Universe and its constituents is one of the greatest remaining puzzles in modern science. Over time, techno-logical advancements have helped in building state-of-the-art telescopes and fast computers, which are being used to uncover novel astrophysical phenom-ena and piece-by-piece phenom-enable solving this puzzle of the formation and evolu-tion of the Universe. Several ground and space-based telescopes have served us in understanding the astrophysics of the local Universe as well as provided strong evidence on the origin and the evolution of the Universe.
The “Big Bang” theory is the most broadly accepted model of the formation of the Universe in the field of astrophysics and cosmology. In this paradigm, the Universe began from a singularity with a grand explosion called the “Big Bang”. The resulting infant Universe was opaque due to the coupling of matter and radiation; hence radiation could not propagate far due to the interaction between photons and baryons (i.e. particles making up ordinary matter). After∼ 380, 000 years, the Universe cooled down sufficiently such that photons and baryons were no longer coupled and radiation escaped into the depths of the Universe, making the latter mostly transparent. This relic radiation is observed as the Cosmic Microwave Background (CMB) radiation in the present Universe.
At this stage, the Universe was mostly neutral, and there were no stars nor galaxies in the universe apart from the fading relic radiation. This period in the history of the Universe is known as the Dark Ages. During the Dark Ages,
Figure 1 – A cartoon depicting 13.8 billion years of the history of the Universe. The inset images show different telescopes, namely Planck, NCLE, LOFAR, JWST and VLT that are being/will be used to probe different eras in the history of the Universe. The background image is adapted from the original graphics of NAOJ.
neutral hydrogen gas accumulated in denser regions due to gravitational insta-bility. Eventually, this gas collapsed to form the first stars and galaxies, which marked the beginning of the Cosmic Dawn (CD). The radiation from the first luminous objects interacted with the neutral hydrogen gas in the surrounding interstellar and intergalactic medium, heating and subsequently ionizing1 it during an era called the Epoch of Reionization (EoR). Our current understand-ing of the Dark Ages, Cosmic Dawn and Reionization eras is primarily based on indirect observations, apart from detections of some rare objects. There are several key questions about these eras that remain unanswered, leaving a hiatus in our current knowledge of the Universe, e.g. What was the exact timing and duration of these eras? When did the first stars form and what were their properties (e.g., size, mass, luminosity)? What were the properties of these first galaxies, e.g., their size, mass, type, morphology, etc.?. It is therefore critical to observe these eras more directly to build a comprehensive understanding of the nature of the first luminous sources in the Universe and their interaction with the surrounding medium. Figure 1 shows a sketch of the timeline of the universe and the instruments which probe the different eras in the history of the Universe.
1 Ionization is a process by which an atom acquires a positive or negative charge by losing
21-cm Cosmology experiments
Hydrogen is the most abundant element in the Universe and makes up around 75% of all baryonic matter. A hydrogen atom, in its neutral state, has a unique characteristic called the “hyperfine transition” which occurs due to changes in alignment of the two spins of the proton and the electron, respectively, in the atom by either absorbing or emitting a photon at a frequency of about 1.4 GHz (i.e., a wavelength of about 21 cm). This spectral feature is also known as the “21-cm line” and was first predicted by the Dutch astronomer Hendrik van de Hulst in 1944 and subsequently detected in observations of the Milky Way in 1951. Since the infant Universe was filled with neutral hydrogen, the 21-cm line, in emission or absorption, is an excellent probe of the physical conditions of the Universe during that early era. Due to the expansion of the Universe, however, the rest-frame wavelength of the 21-cm line is increased (i.e., “redshifted”). In present-day Universe, the wavelength of the redshifted 21-cm radiation from the Dark Ages, the CD and the EoR falls in the range of several meters, with a corresponding frequency range of about 50 to 200 MHz. Each frequency in this range corresponds to a distinct epoch in time in the infant Universe. Observing the redshifted 21-cm radiation at different wavelengths allows us to construct a three-dimensional representation of the evolving Universe.
Several observational efforts are undertaken (or planned) around the Globe to measure this redshifted 21-cm signal from the CD and EoR. These “21-cm experiments” can be broadly classified into two categories: global-signal ex-periments and power-spectrum exex-periments. The global signal exex-periments aim to study the global (sky-averaged) properties of the infant Universe and how these properties evolve with Cosmic time. These experiments utilise rela-tively simple instruments, e.g. single dipoles, which observe nearly the entire sky at once at a given time. On the other hand, power spectrum experiments focus on measuring the spatial variations in the 21-cm signal. Power spectrum experiments require much larger and more sensitive instruments, e.g. large arrays of radio receivers called interferometers.
Observational challenges in 21-cm Cosmology
The redshifted 21-cm signal from the CD and EoR is extremely faint. More-over, this weak signal gets contaminated by bright radiation from intervening galaxies and our own Milky Way (also known as “foreground radiation”) which is thousands of times brighter than the 21-cm signal itself. Besides this, the upper layer of Earth’s atmosphere, called ionosphere, distorts the electromag-netic radiation propagating through it. In addition, Radio-Frequency Inter-ference (RFI) from man-made sources such as television receivers, FM radio,
nal. Even the most sensitive telescopes at present require thousands of hours of sky exposure, incredibly precise signal corrections (called “calibration”) of the telescope, and accurate removal of these contaminants to detect the signal.
21-cm Cosmology with LOFAR and AARTFAAC
LOFAR (i.e., the Low-Frequency ARray) is a new generation state-of-the-art radio telescope (interferometer) built in the Netherlands. LOFAR is designed to observe the sky in the 30-200 MHz frequency range. Due to its unique design, the LOFAR telescope is incredibly sensitive and is capable of measuring the redshifted 21-cm signal from the infant Universe. The LOFAR-EoR key science project aims to measure the power spectrum of the redshifted 21-cm signal from the EoR. The LOFAR-EoR research team is working towards understanding many of the observational challenges and signal contaminants, such as the bright foregrounds, ionospheric effects, RFI, data processing and analysis. The final goal of the team is detecting the faint cosmological 21-cm signal. In addition to this, the LOFAR-EoR team has also commenced a long-term observing programme called AARTFAAC Cosmic Explorer (ACE) using the AARTFAAC telescope. AARTFAAC (Amsterdam-ASTRON Radio Transients Facility And Analysis Center) is a LOFAR based telescope which is capable of imaging the full-sky, in its LBA mode, rather than being limited to a small region of the sky while maintaining the sensitivity of LOFAR. This capability of AARTFAAC makes it a suitable instrument to measure large scale spatial variations in the redshifted 21-cm signal. The work I have presented in this thesis mainly focuses on understanding the observational challenges in 21-cm Cosmic Dawn experiments such as the LOFAR EoR KSP and ACE, and their implications for next-generation telescopes such as the Square Kilometer Array (SKA), the Hydrogen Epoch of Reionization Array (HERA) and NENUFAR, which will focus on observing the redshifted 21-cm signal from the CD epoch.
This thesis
In Chapter 2, I discuss a number of observational challenges in Cosmic Dawn experiments using the LOFAR Low Band Antenna (LBA) observations of a region of the sky. The contamination effects become more severe at ‘lower frequencies’ and need to be removed or mitigated to detect the redshifted 21-cm signal. I use various statistical techniques to quantify these contamination effects and propose different mitigation methods.
In Chapter 3, I present a unique scheme to calibrate2 the two sky regions observed simultaneously with LOFAR-LBA and also implemented the lessons learnt in Chapter 2 to improve the quality of data processing and analysis. I also demonstrate the use of a foreground removal technique called Gaussian Process Regression to remove the foregrounds from the observed data. Finally, I measure a power spectrum upper limit on the 21-cm signal from the processed data and found that the weak 21-cm signal from the Cosmic Dawn has an amplitude below 14 Kelvin.
In Chapter 4, I present the first results from the data observed in the ACE programme. I demonstrate the end-to-end use of the LOFAR data processing pipeline on 6 hours of ACE data. ACE investigates the sky in a particular mode different from LOFAR. Therefore, I present a modified methodology to process and analyse such data. From the first results, I conclude that ACE has the potential to detect the 21-cm signal from the Cosmic Dawn, provided that the observational challenges are mitigated/circumvent.
In Chapter 5, I present first images from High Band Antenna (HBA) mode of the AARTFAAC system. I find that the images produced with AARTFAAC-HBA are dominated by diffuse radiation emitted by electrons accelerating in a magnetic field (synchrotron radiation). I present two techniques to model compact sources and this diffuse radiation and show how they compare with each other. I conclude that a hybrid approach which combines the two methods is needed to produce a more accurate representation of the sky which in turn will help in improving the quality of the calibration process.
Although the 21-cm signal still remains elusive, the techniques that I have presented and the lessons learnt in this thesis will prove to be useful in miti-gating/circumventing the observational challenges in experiments to measure this cosmological signature.
2 Calibration refers to the process of correcting errors in the measurements by comparing
