The First Galaxies in the Universe, Abraham Loeb and Steven R. Furlanetto, Princeton U. Press, 2013. $130.00 (540 pp.). ISBN 978-0-691-14491-7
Anyone who loves galaxies will eventually wonder where they came from. And like archaeologists digging the earliest human hearths and temples out of the Neolithic dust, astronomers today are nearing the cosmic strata where the first galaxies are to be found.
Ninety years after Edwin Hubble discovered that many faint nebulae are in fact galaxies—“island universes” like our own Milky Way—his namesake telescope detected galaxies formed when the universe was only 2% of its present age. Yet the pale red dots seen by Hubble are only the most conspicuously luminous of galaxies at their epoch; most galaxies formed at that time are too faint to see with our current telescopes. But change is coming. Over the next decade, the James Webb Space Telescope (JWST), the Atacama Large Millimeter/Submillimeter Array, and 30-meter-class telescopes on the ground should see far enough back in time to observe the first building blocks of today’s typical galaxies. Until then, thinking about the first galaxies is still a theory-heavy enterprise.
Enter Abraham Loeb and Steven Furlanetto, two of the most consistently active theorists on the subject. Their new textbook, The First Galaxies in the Universe, purports to be “a comprehensive, self-contained introduction” to its topic and aims to “bridge the gap between theory and observation.” The book excels on the first of those ambitious goals and—but for a few significant omissions—succeeds on the second.
The First Galaxies in the Universe approaches its subject from a pedagogical distance. The first four chapters are devoted to the basics of structure formation out of the intergalactic medium (IGM). Though highly overlapping with many other introductory texts, the foundational material builds logically and at a suitable level of mathematical detail toward the specialized content in the last third of the book, even as it covers all the important basic concepts. While perusing those early sections, I particularly appreciated the obvious care taken by the authors and their publisher in selecting the supporting figures and in homogenizing their format and appearance in order to express the key points without needless distraction.
The book hits its stride in its final third with extended treatments of the reionization of the IGM, optical and IR surveys for high-redshift galaxies (such as by Hubble and eventually JWST), and two promising techniques for probing the first galaxies and the IGM gas that fuels their star formation. Those techniques are based on the Lyman-alpha electronic transition of hydrogen and the hyperfine spin flip between energy levels that produces the 21-cm line. The authors were among the early advocates and developers of the 21-cm-line method, and they are eminently qualified to produce a worthy source of pedagogical material on the theory of both techniques.
The 21-cm line promises to provide a nearly complete map of the normal matter in the universe over large swaths of sky prior to and during reionization, once new radio facilities such as LOFAR in the Netherlands and the Square Kilometre Array in Australia and South Africa are fully operational. The Lyman-alpha line arises from reforming hydrogen that had been ionized by radiation from the first galaxies, and so it probes the interaction between stars and their surrounding gas.
However, the complexity of interpreting the Lyman-alpha and 21-cm signals belies their humble origins in simple transitions of nature’s most abundant element. The many radiative-transfer effects that alter the shape and strength of the lines as the photons pass from the galaxies, through the IGM, and into our telescopes are covered in enough detail to equip readers with a good working understanding of how the lines are produced and interpreted. That material is a valuable resource for students entering the field and looking ahead to the day, sometime later in this decade, when new facilities will apply those techniques on a massive scale.
True to its aim, the book introduces the relevant observations throughout. Particularly in its discussion of the Lyman-alpha and 21-cm techniques, the promised bridge to observations is soundly built and, for a fast-moving field, impressively up to date. By contrast, other promising techniques for detecting and understanding the first galaxies receive only bullet-point treatment. Just the last seven pages of the text concern the rich “fossil record” of the first heavy elements and the early mass function of stars preserved within long-lived stars in the nearby universe. The “ultra-faint” dwarf galaxies on the outskirts of the Milky Way, believed by many astronomers to be preserved remnants of the first galaxies themselves, are mentioned only in passing. Coverage of constraints from microwave and IR radiation backgrounds is also cursory. That matters because all those approaches provide insight into physical processes that are critical to the formation of the first galaxies but invisible in the hydrogen emission that is so thoroughly described.
Students looking for an introduction to complementary avenues to the first galaxies will have to look elsewhere. But despite those omissions—the book’s only real flaw—graduate students or senior undergraduates will find The First Galaxies in the Universe a thorough introduction to the topic. Interested professionals will find it a helpful entry point to the specialist literature on one of the most exciting frontiers in astrophysics.
Jason Tumlinson is an astronomer at the Space Telescope Science Institute in Baltimore, Maryland. He studies the formation and evolution of stars and galaxies over cosmic time.
