Astronomers use distant objects as flashlights to peer through the vast space between galaxies, the so-called intergalactic medium (IGM). Intervening gas clouds absorb light in particular atomic transitions, producing an absorption spectrum, which can then be used to study the evolution of IGM properties throughout cosmic time. The first quasi-stellar objects (known as QSOs or quasars) were identified in 1960 by Rudolph Minkowski 1 [1], Allan Sandage, and Maarten Schmidt [2]. They had star-like images, strong radio emission, and strangely placed, broad emission lines. Schmidt realized a few years later that the emission lines were actually hydrogen Balmer (principle quantum number n → 2) series lines redshifted by 16% [3], who concluded that an extragalactic origin was the “most direct and least objectable” explanation. This realization sparked a flurry of research on QSOs (e.g. [4, 5, 6, 7]), which were proposed to be powered by supermassive black holes (SMBHs) in 1964 by Edwin Salpeter [8] and Yakov Zel’dovich [9], and later associated with galactic nuclei by Donald Lynden-Bell [10] in 1969. These ideas were slowly accepted, but mounting evidence, especially with X-ray observations in the following decade (e.g. [11]), confirmed the black hole paradigm.
James Gunn and Bruce Peterson [12] first realized in 1965 that even if a tiny fraction (∼ 10−4) of the IGM was neutral it would absorb the QSO light blueward of the Lyα wavelength at 1216 Å (n = 1 → 2). However when interpreting the most distant QSO at the time (redshifted by a factor of 2.01), there was no such absorption, strongly suggesting that the IGM was highly ionized when the universe was only one-quarter of its present age of 13.8 billion years.
Cosmic reionization is the process in which the IGM becomes ionized and heated. Now from many observations of the IGM and early galaxies, we know that reionization 2 occurs everywhere in the IGM within a billion years after the Big Bang. As the first generations of galaxies fiercely form, they provide the necessary radiation to propel this cosmological event. Thus, it is informative to first overview some cosmological concepts and the astrophysics of photo-ionization in order to obtain a fuller understanding of reionization.
1 Son of Hermann Minkowski of the ’Minkowski spacetime’ in general relativity. Back.
2 In this article, we refer to ’reionization’ as the reionization of hydrogen and singly-ionized helium. Helium is doubly-ionized at a time 2–3 billion years after the Big Bang. Back.