The First Nonlinear Structures and the Reionization History of the Universe

In cosmological models favored by current observations, the first astrophysical objects formed in dark matter halos at redshifts starting at z>20, and their properties were determined by primordial H_2 molecular chemistry. These protogalaxies were very abundant, but substantially less massive than typical galaxies in the local Universe. Extreme metal-poor stars, and massive black holes in their nuclei reionized the bulk of the hydrogen in the intergalactic medium. Reionization may have taken place over an extended redshift interval, ending around z=7. Observational probes of the process of reionization are afforded by studying the polarization of the cosmic microwave background anisotropies, as well as by studying the spectra and abundance of distant Lyman alpha-emitting galaxies. Here we review theoretical expectations on how and when the first galaxies formed, and summarize future observational prospects of probing hydrogen reionization.