The Intergalactic Medium at High Redshifts

Abstract

The intergalactic medium (IGM) contains >95% of the mass in the Universe at high redshifts, and its properties control the earliest phases of structure formation and the reionization process. Although its evolution may seem straightforward, a number of feedback mechanisms can dramatically affect it. Radiative feedback, through a Lyman-Werner background, an X-ray background, and photoionization, affect halo collapse and the clumping of the IGM. We describe how the redshifted 21 cm background can be used to study these effects. Chemical feedback, primarily through supernova winds, changes the modes of star formation and halo cooling; it can be studied through metal absorption lines with the JWST, as well as metal lines in the cosmic microwave background, direct observations of cooling radiation, and fossil evidence in the nearby Universe. Finally, we describe how uncertainties in our modeling of the IGM structure affect reionization models and observations. Detailed studies of helium reionization, which occurs at the much more accessible z∼3, will significantly improve these models over the next few years.