Search Results

This chapter examines the intergalactic medium (IGM). Although much of astronomy focuses on the luminous material inside galaxies, the majority of matter today—and the vast majority at z > 6—actually lies outside these structures, in the IGM. This material ultimately provides the fuel for galaxy and cluster formation and—because it is much less affected by the complex physics of galaxies—offers a cleaner view of the underlying physical processes of structure formation and of fundamental cosmology. The chapter thus takes up the study on the properties of the IGM, especially during the era of the first galaxies (when the IGM underwent major changes).

This chapter investigates a number of specific observational probes of the high-redshift Universe. It examines the Lyman-α‎ line, an extraordinarily rich and useful—albeit complex—probe of both galaxies and the intergalactic medium (IGM). As established in the previous chapter, young star-forming galaxies can produce very bright Lyman-α‎ emissions. Although the radiative transfer of these photons through their host galaxies is typically very complex, a good starting point is a simple model in which a fraction of stellar ionizing photons are absorbed within their source galaxy, forming embedded H II regions. The resulting protons and electrons then recombine, producing Lyman-α‎ photons. Assuming ionization equilibrium, the rate of these recombinations must equal the rate at which ionizing photons are produced.

This chapter describes how the 21-cm line is used to study the high-z Universe. It introduces the spin-flip or the hyperfine line—a transition driven by the interaction of the spins of the proton and electron, whose relative directions affect the energy of the electron's orbit. An atom in the upper state eventually undergoes a spin-flip transition, emitting a photon with a wavelength of 21 cm. As the chapter shows, this transition is extremely weak, so the effective intergalactic medium (IGM) optical depth is only of the order of 1 percent: this makes the entire neutral IGM accessible during the cosmic dawn. Moreover, the transition energy is so low that it provides a sensitive thermometer of the low-temperature IGM, and as a low-frequency radio transition, it can be seen across the entirety of the IGM against the cosmic microwave background.