Preface

I find galaxies absolutely fascinating. The scales of galaxies are hard to fathom. Each galaxy is thousands to hundreds of thousands of lightyears wide, containing billions upon billions of stars and healthy helpings of gas and dark matter. And billions of large galaxies like the Milky Way exist in the observable Universe together with trillions more smaller galaxies. Galaxies we observe display an enormous diversity of beautiful patterns; no galaxy is like another. And yet, these galaxies emerge from the tiniest seeds right after the Big Bang—likely seeded by quantum fluctuations in the primordial soup—that grow under the force of gravity into the large, massive objects that we see today. Galaxies are fascinating, because they are some of the most beautiful, majestic, and complex objects in nature, and we can understand them using fundamental physics. And because they form and evolve in the expanding Universe starting at the Big Bang, they are directly and inextricably linked to some of the greatest mysteries in Physics: the nature of the dark matter that dominates the matter budget in the Universe as a whole and within individual galaxies, the nature of Dark Energy that sets the cosmological backdrop within which galaxies evolve, and the (perhaps inflationary) origin of the tiny galaxy seeds at the Big Bang. This book’s overarching aim is to provide you with a deep, modern understanding of the structure of galaxies, how it is shaped primarily through gravity, and how galaxies form, grow, and evolve in the expanding Universe.

This book originated as a set of notes written to accompany a semester-long course on “Galactic Structure and Dynamics” at the University of Toronto that I started teaching in the Fall of 2017 and have since taught three more times. Like most people writing an advanced textbook, I started writing notes because none of the existing texts approached the topic anywhere close to the manner in which I wished to teach about galaxies. Excellent graduate-level textbooks on galaxies existed at the time (foremost “Galactic Dynamics” by Binney & Tremaine 2008, often accompanied by Binney & Merrifield 1998 or nowadays Mo et al. 2010), but I found myself having to constantly jump around within and between books to create a modern introduction to galaxies. This book aims to introduce galaxies in a logical, pedagogical manner starting from simple considerations and building up towards a more sophisticated understanding of the different types of galaxies, in a way that at least in the first three parts could be taught linearly following the structure of the book (but leaving options open after Part I to select topics and skip around if desired). At the same time, I aim to connect to interesting applications throughout the book, even in the simple treatment of Part I.

The second objective of this book is reduce the distance between a standard textbook and the practice of research. This book is accompanied by a website, https://galaxiesbook.org, that contains code examples for essentially all of the figures made in the book that are not taken from previous work. All of these examples can be run in python on your own machine or directly in the browser using cloud computing; you should play around with these, changing parameters in the code to see what happens and deepen your understanding. Some of my favorites are the two-dimensional velocity fields in Chapter 8.1, the surfaces of section and chaos in Chapter 13, the Schwarzschild modeling example in Chapter 14.3.2, the example tree-based \(N\)-body solver in Chapter 12.4.1.2, and the galaxy merger trees in Chapter 19.2.2 (but there are many more!). Many of the examples use galpy, a Python-based galactic-dynamics code (Bovy 2015). The online version of this book also contains interactive visualizations of data sets (such as the \((l,v)\) diagram of molecular clouds in Chapter 8.3.1) and of orbits computed in galpy (e.g., orbits in an axisymmetric potential in Chapter 9.1 and box and loop orbits in non-axisymmetric potentials in Chapter 13). The latter hopefully give a much better sense of the dynamic nature of the subject than the static images of orbits that one typically finds in books and journal articles!

This book can most straightforwardly be used for graduate courses on the structure of galaxies with an emphasis on their dynamical structure. Such courses start by going through the introduction and essentially all of Part I (perhaps skipping sections on action-angle coordinates and anisotropic distribution functions) and covering the first three chapters of Part III on disk potentials, disk kinematics, and orbits in disks. At that point, such a course could choose to get more into the dynamical structure of galaxies by diving deeper into equilibrium dynamical models of disks (Chapter 10) and/or the structure, gravitational potentials, orbits, and internal dynamics of elliptical galaxies covered in Part III. Or such a course could decide to broaden into topics of galaxy evolution, covering selected topics in chemical evolution (Chapter 11), the growth of structure (Chapter 17) , galaxy classification, scaling laws, and galactic star formation (Chapter 18), and/or hierarchical galaxy formation (Chapter 19). More advanced courses in galactic dynamics will not want to skip the discussion of general Poisson solvers (including \(N\)-body modeling) in Chapter 12.3 and 12.4 or the detailed discussion of chaos and orbits in triaxial mass distributions in Chapter 13 and they will enjoy the deep dives into dynamical friction and tides from Chapter 19.4 and into disk stability, bars, and spiral structure in Chapter 20 (the length of these treatments reflects my own pleasure in studying them!). Any course on galaxies will benefit from a fun detour into the basics of gravitational lensing (Chapter 15).

This book is less obviously suited for courses in extragalactic astronomy, but it could nevertheless serve as a basis for such a course. The introduction can serve as a gentle entrance to the world of galaxies, followed by a more in-depth discussion of the different types of galaxies in Chapter 18.3 and of galaxy scaling relations and galactic star formation in Chapter 18.4 and 18.5. To understand how essential galactic properties are inferred from observations, a course in extragalactic astronomy will also want to discuss stellar-population synthesis (Chapter 18.1). To connect galaxies to cosmology, such a course can cover the growth of structure in the Universe, gravitational collapse, and virialization (Chapter 17), as well as the halo mass function (Chapter 17.4) and its connection to the galaxy luminosity and stellar-mass functions (Chapter 18.2). A deeper treatment of galaxy evolution can consist of the discussion of the feedback-regulated growth of galaxies through mergers and gas accretion discussed in Chapter 19 as well as the observational properties and effects of bars and spirals discussed in Chapter 20.2 and 20.3.

This book assumes a familiarity with math and, in particular, calculus at the level of an advanced undergraduate student in Physics or Astronomy; some mathematical background is discussed in Appendix B, but the purpose of that Appendix is not to give a comprehensive overview of the required mathematical background.

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