Cool start to hydrogen ionization

Abstract

Simulations of the cosmos cast doubt on assumptions about the temperature of primordial hydrogen gas when it was ionized by the first stars and galaxies, complicating the interpretation of ongoing observations. See Letter p.197 The epoch of reionization was a one during which the neutral gas of the 'dark age' Universe became an ionized plasma. Simulations suggest that the 21-cm spectral transition of atomic hydrogen will show a clear fluctuation peak at a redshift and scale that mark the central stage of reionization. This prediction is based on the assumption that heating of the cosmic gas, which occurs before reionization, is caused by stellar remnants (particularly X-ray binaries) and reaches temperatures well above the cosmic microwave background of the time (30 kelvin). However, Rennan Barkana and colleagues report that the hard spectra of X-ray binaries (with more high-energy than low-energy photons) make such heating ineffective, resulting in a delayed and spatially uniform heating. In this new model, the 21-cm signature of reionization is modified to a more complex signal with a distinct minimum temperature (below 1 millikelvin) that marks the rise of the cosmic mean gas temperature above the microwave background.