Radio Signatures of Hiat High Redshift: Mapping the End of the “Dark Ages”

The emission of 21-cm radiation from a neutral intergalactic medium (IGM) athigh redshift is discussed in connection with the thermal and ionizationhistory of the universe. The physical mechanisms that make such radiationdetectable against the cosmic microwave background include Ly_alpha coupling ofthe hydrogen spin temperature to the kinetic temperature of the gas andpreheating of the IGM by the first generation of stars and quasars. Threedifferent signatures are investigated in detail: (a) the fluctuations in theredshifted 21-cm emission induced by the gas density inhomogeneities thatdevelop at early times in cold dark matter (CDM) dominated cosmologies; (b) thesharp absorption feature in the radio sky due to the rapid rise of the Ly_alphacontinuum background that marks the birth of the first UV sources in theuniverse; and (c) the 21-cm emission and absorption shells that are generatedon several Mpc scales around the first bright quasars. Future radioobservations with projected facilities like the Giant Metrewave Radio Telescopeand the Square Kilometer Array may shed light on the power spectrum of densityfluctuations at z>5, and map the end of the "dark ages", i.e. the transitionfrom the post-recombination universe to one populated with radiation sources.Comment: LateX, 19 pages, 5 figures, significantly revised version to be published in the Ap