Detecting the Earliest Galaxies through Two New Sources of 21 Centimeter Fluctuations

The first galaxies that formed at a redshift ~20-30 emitted continuum photonswith energies between the Lyman-alpha and Lyman limit wavelengths of hydrogen,to which the neutral universe was transparent except at the Lyman-seriesresonances. As these photons redshifted or scattered into the Lyman-alpharesonance they coupled the spin temperature of the 21cm transition of hydrogento the gas temperature, allowing it to deviate from the microwave backgroundtemperature. We show that the fluctuations in the radiation emitted by thefirst galaxies produced strong fluctuations in the 21cm flux before theLyman-alpha coupling became saturated. The fluctuations were caused by biasedinhomogeneities in the density of galaxies, along with Poisson fluctuations inthe number of galaxies. Observing the power-spectra of these two sources wouldprobe the number density of the earliest galaxies and the typical mass of theirhost dark matter halos. The enhanced amplitude of the 21cm fluctuations fromthe era of Lyman-alpha coupling improves considerably the practical prospectsfor their detection.Comment: 11 pages, 7 figures, ApJ, published. Normalization fixed in top panels of Figures 4-