The Physics and Early History of the Intergalactic Medium

The intergalactic medium – the cosmic gas that fills the great spaces between the galaxies – is affected by processes ranging from quantum fluctuations in the very early universe to radiative emission from newly-formed stars. This gives the intergalactic medium a dual role as a powerful probe both of fundamental physics and of astrophysics. The heading of fundamental physics includes conditions in the very early universe and cosmological parameters that determine the age of the universe and its matter content. The astrophysics refers to chapters of the long cosmic history of stars and galaxies that are being revealed through the effects of stellar feedback on the cosmic gas. This review describes the physics of the intergalactic medium, focusing on recent theoretical and observational developments in understanding early cosmic history. In particular, the earliest generation of stars is thought to have transformed the universe from darkness to light and to have had an enormous impact on the intergalactic medium. Half a million years after the big bang the universe was filled with atomic hydrogen. As gravity pulled gas clouds together, the first stars ignited and their radiation turned the surrounding atoms back into free electrons and ions. From the observed spectral absorption signatures of the gas between us and distant sources, we know that the process of reionization pervaded most of space a billion years after the big bang, so that only a small fraction of the primordial hydrogen atoms remained between galaxies. Knowing exactly when and how the reionization process happened is a primary goal of cosmologists, because this would tell us when the early stars and black holes formed and in what kinds of galaxies. The distribution and clustering of these galaxies is particularly interesting since it is driven by primordial density fluctuations in the dark matter. Cosmic reionization is beginning to be understood with the help of theoretical models and computer simulations. Numerical simulations of reionization are computationally challenging, as they require radiative transfer across large cosmological volumes as well as sufficiently high resolution to identify the sources of the ionizing radiation in the infant universe. Rapid progress in our understanding is expected with additional observational input. A wide variety of instruments currently under design – including large-aperture infrared telescopes on the ground or in space (JWST ), and low-frequency radio telescope arrays for the detection of red shifted 21-cm radiation – will probe the first sources of light during an epoch in cosmic history that has been largely unexplored so far. The new observations and the challenges for theoretical models and numerical simulations will motivate intense work in this field over the coming decade. PACS numbers: 95.85.Bh, 97.20.Wt, 98.54.Kt, 98.62.Ai, 98.62.Ra, 98.65.Dx, 98.80.Bp, 98.80.Es The Physics and Early History of the Intergalactic Medium 2 Figure 1. Cosmology is like archeology. The deeper one looks, the older is the layer that is revealed, owing to the finite propagation speed of light (illustration from Loeb 2006).