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The first galaxies that formed at a redshift z similar to 20-30 emitted continuum photons with energies between the Ly alpha and Lyman limit wavelengths of hydrogen, to which the neutral universe was transparent except at the Lyman series resonances. As these photons redshifted or scattered into the Ly alpha resonance, they coupled the spin temperature of the 21 cm transition of hydrogen to the gas temperature, allowing it to deviate from the microwave background temperature. We show that the fluctuations in the radiation emitted by the first galaxies produced strong fluctuations in the 21 cm flux before the Ly alpha coupling became saturated. The fluctuations were caused by biased inhomogeneities in the density of galaxies, along with Poisson fluctuations in the number of galaxies. Observing the power spectra of these two sources would probe the number density of the earliest galaxies and the typical mass of their host dark matter halos. The enhanced amplitude of the 21 cm fluctuations from the era of Ly alpha coupling improves considerably the practical prospects for their detection.
Barkana et al. (Thu,) studied this question.