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The Epoch of Reionization (hereafter "cosmic reionization" or simply "reionization") is one of the most significant remaining puzzles in cosmic history. Occurring less than one billion years after the Big Bang, reionization likely took place shortly after the formation of the first baryonic structures such as stars, galaxies and active galactic nuclei (AGN). Reionization is therefore intimately linked to several core astronomical fields such as: stellar formation and evolution, galaxy formation and evolution, large scale structure, and the cosmic microwave background, to name a few. In the last decade, significant investments have been made to observe galaxies during reionization, for example in the optical and near infrared with the Hubble Ultra-Deep Field, the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey, and the Hubble Frontier Fields. Experiments at radio frequencies have recently been undertaken to detect the global 21 cm signal expected from reionization, for example the Experiment to Detect the Global Epoch of reionization Signature, as well as to measure the power spectrum of the 21 cm radiation, such as the Murchison Wide Field Array and the Low-Frequency Array. While reionization has been a major focus in astronomy for over a decade, many key properties of the process are still uncertain. The main difficulty in constraining reionization is the faintness of the sources likely driving it, i.e. the first galaxies and AGN. Even with the most powerful current telescopes such as Keck and the Hubble Space Telescope (HST), observing all but the brightest of these sources is too expensive. Using a galaxy cluster as a gravitational lens, this observational challenge can be somewhat eased. In order to take advantage of cluster lensing to study faint galaxies, a gravitational lens model is required. In this dissertation, I present gravitational lens models I developed for 11 galaxy clusters in three large cluster surveys undertaken with the Hubble Space Telescope and the Spitzer Space Telescope. I selected redshift (z) 7 to 8 galaxies behind all of these clusters and followed them up with the Multi-Object Spectrometer For InfraRed Exploration (MOSFIRE) on the Keck I telescope. During this spectroscopic campaign I discovered a Lyman-alpha emitting galaxy at z=7.64 magnified by a factor of approximately 10. Its intrinsic luminosity is an order of magnitude fainter than the handful of other known Lyman-alpha emitters (LAEs) at z>7.5, all of which are bright, rare sources. While exhibiting weaker Lyman-alpha than the UV-brighter LAEs at z>7.5, the underlying mechanism which allows Lyman-alpha to escape all of these galaxies may be the same. I also obtained the tightest spectroscopic constraints on the redshift of one of the highest redshift gravitationally lensed galaxies (z~9) using ultra-deep grism spectroscopy from the Hubble Space Telescope. Using a sub-sample consisting of 8 out of 11 of these clusters for which the photometric analysis is complete, I constrained the fraction of Lyman-break galaxies showing Lyman-alpha, often just called the "Lyman-alpha fraction test," at z=8, making the first definitive measurement of a declining Lyman-alpha fraction (and hence increasing neutral hydrogen fraction) over the interval z=7-8 for UV-fainter galaxies. This is the first hint that faint galaxies suffer a similarly large decline in Lyman-alpha fraction as bright galaxies over this interval, suggesting that the overlap phase of reionization may still be underway at z=8.
Documents how astronomers are learning from objects in the universe that are on the edge of visibility, and discusses how these objects are informing the scientific community about the beginnings of time and other mysteries.
A concise introduction to cosmology and how light first emerged in the universe Though astrophysicists have developed a theoretical framework for understanding how the first stars and galaxies formed, only now are we able to begin testing those theories with actual observations of the very distant, early universe. We are entering a new and exciting era of discovery that will advance the frontiers of knowledge, and this book couldn't be more timely. It covers all the basic concepts in cosmology, drawing on insights from an astronomer who has pioneered much of this research over the past two decades. Abraham Loeb starts from first principles, tracing the theoretical foundations of cosmology and carefully explaining the physics behind them. Topics include the gravitational growth of perturbations in an expanding universe, the abundance and properties of dark matter halos and galaxies, reionization, the observational methods used to detect the earliest galaxies and probe the diffuse gas between them—and much more. Cosmology seeks to solve the fundamental mystery of our cosmic origins. This book offers a succinct and accessible primer at a time when breathtaking technological advances promise a wealth of new observational data on the first stars and galaxies. Provides a concise introduction to cosmology Covers all the basic concepts Gives an overview of the gravitational growth of perturbations in an expanding universe Explains the process of reionization Describes the observational methods used to detect the earliest galaxies
The formation of the first stars (Pop III stars) and galaxies is one of the great outstanding challenges in modern astrophysics and cosmology. The first stars are likely key drivers for early cosmic evolution and will be at the center of attention over the next decade. The best available space and ground-based telescopes like the Hubble Space Telescope probe the Universe to high redshifts and provide us with tantalizing hints; but they cannot yet directly detect the first generation of stars and the formation of the first galaxies. This is left as key science for future telecopes like the James Webb Space Telescope. This book is based in part on classroom tested lectures related to Pop III stars, but also draws from the author's review articles of the main physical principles involved. The book will thus combine pedagogical introductory chapters with more advanced ones to survey the cutting-edge advances from the frontier of research. It covers the theory of first star formation, the relation between first stars and dark matter, their impact on cosmology, their observational signatures, the transition to normal star formation as well as the assembly of the first galaxies. It will prepare students for interpreting observational findings and their cosmological implications.
The dawn of the first stars, galaxies and black holes signaled a fundamental milestone in our Universe’s evolution: the Epoch of Reionization. The light from these galactic ancestors began spreading out, ionizing virtually every atom in existence. Our Universe transitioned from darkness to light, from cold to hot, from simple and boring to the wondrous cosmic zoo we see around us today. Despite its importance, observations of reionization have been few, and their interpretation has been highly controversial. Fortunately, this is rapidly changing. We will soon enter the "Big Data” era of this mysterious epoch, driven by an upcoming wave of observations with state-of-the-art telescopes as well as new sophisticated analysis tools. The aim of this volume is to summarize the current status and future outlook of the reionization field. We bring together leading experts in many sub-disciplines, highlighting the measurements that will illuminate our understanding of reionization and the cosmic dawn: (i) 21cm interferometry; (ii) high-redshift quasar spectra; (iii) high-redshift galaxy surveys; (iv) primary and secondary anisotropies of the Cosmic Microwave Background; (v) high-resolution studies of the metal content of early galaxies. We seek a roadmap to interpreting the wealth of upcoming observations. What is the best use of limited observational resources? How do we develop theoretical tools tailored for each observation? Ultimately, what will we learn about the epoch of reionization and our galactic ancestors?
The universe we live in is expanding faster and faster. This phenomenon called cosmic acceleration is one of the most puzzling cosmological discoveries in the past 25 years: even the least exotic explanation requires a new pervasive energy component in our universe (called dark energy). Despite the mysterious nature of dark energy, a model ($\Lambda$CDM) based on Einstein's general relativity, a cosmological constant (a specific form of dark energy), and slowly moving dark matter, seems to be able to describe a variety of observations from the high- to low-redshift universe. To understand the nature of dark energy and to test the $\Lambda$CDM paradigm, ambitious cosmological surveys, such as the Dark Energy Survey (DES), the Dark Energy Spectroscopic Instrument (DESI), the Rubin Observatory's Legacy Survey of Space and Time (LSST), and the Roman Space Telescope, aim to precisely and robustly measure cosmic structure and its evolution via various cosmological probes, such as weak gravitational lensing, galaxy clustering, and other techniques. Combining multiple cosmological probes (known as multi-probe analyses) provides precise and robust cosmological constraints. Galaxy clustering, weak gravitational lensing, and abundances of galaxy clusters each are sensitive to different aspects of cosmic structure formation and are affected by different astrophysical and observational uncertainties. Thus, their combination is expected to be more precise and robust than any of the probe alone. Among these probes, the abundances and spatial distribution of galaxy clusters, which are associated with the highest peaks in the matter density field, provide powerful probes of cosmic structure and its evolution. This thesis presents original research that improves our understandings of the universe by observations of galaxy clusters. In the three self-contained projects, I (1) develop and validate methods for combining cluster abundances and two-point correlation functions, (2) perform the first blind cosmology analysis on combining cluster abundances, weak gravitational lensing, and galaxy clustering using data taken in the first season (DES-Y1) of the Dark Energy Survey, and (3) quantify the connections between red galaxies and their host dark matter halos by modeling luminosity functions of galaxies in galaxy clusters. While these three projects have already advanced our understandings of the cosmos, they also serve as an example of how one can use millions of clusters expected to be detected with the upcoming surveys in 2020s to improve our knowledge of the universe. These opportunities are also discussed in this thesis.
Driven by discoveries, and enabled by leaps in technology and imagination, our understanding of the universe has changed dramatically during the course of the last few decades. The fields of astronomy and astrophysics are making new connections to physics, chemistry, biology, and computer science. Based on a broad and comprehensive survey of scientific opportunities, infrastructure, and organization in a national and international context, New Worlds, New Horizons in Astronomy and Astrophysics outlines a plan for ground- and space- based astronomy and astrophysics for the decade of the 2010's. Realizing these scientific opportunities is contingent upon maintaining and strengthening the foundations of the research enterprise including technological development, theory, computation and data handling, laboratory experiments, and human resources. New Worlds, New Horizons in Astronomy and Astrophysics proposes enhancing innovative but moderate-cost programs in space and on the ground that will enable the community to respond rapidly and flexibly to new scientific discoveries. The book recommends beginning construction on survey telescopes in space and on the ground to investigate the nature of dark energy, as well as the next generation of large ground-based giant optical telescopes and a new class of space-based gravitational observatory to observe the merging of distant black holes and precisely test theories of gravity. New Worlds, New Horizons in Astronomy and Astrophysics recommends a balanced and executable program that will support research surrounding the most profound questions about the cosmos. The discoveries ahead will facilitate the search for habitable planets, shed light on dark energy and dark matter, and aid our understanding of the history of the universe and how the earliest stars and galaxies formed. The book is a useful resource for agencies supporting the field of astronomy and astrophysics, the Congressional committees with jurisdiction over those agencies, the scientific community, and the public.
This book provides a comprehensive, self-contained introduction to one of the most exciting frontiers in astrophysics today: the quest to understand how the oldest and most distant galaxies in our universe first formed. Until now, most research on this question has been theoretical, but the next few years will bring about a new generation of large telescopes that promise to supply a flood of data about the infant universe during its first billion years after the big bang. This book bridges the gap between theory and observation. It is an invaluable reference for students and researchers on early galaxies. The First Galaxies in the Universe starts from basic physical principles before moving on to more advanced material. Topics include the gravitational growth of structure, the intergalactic medium, the formation and evolution of the first stars and black holes, feedback and galaxy evolution, reionization, 21-cm cosmology, and more. Provides a comprehensive introduction to this exciting frontier in astrophysics Begins from first principles Covers advanced topics such as the first stars and 21-cm cosmology Prepares students for research using the next generation of large telescopes Discusses many open questions to be explored in the coming decade