Line-of-Sight Anisotropy of the 21cm Fluctuations Prior to Reionization
Abstract
Between redshifts ~200 and ~25, the adiabatically-cooling cosmic hydrogen absorbed the warmer microwave background at its resonant 21cm spin-flip transition. Subsequently, the gas was heated by radiation from the first galaxies and began to emit excess flux at this wavelength. Fluctuations in the 21cm flux were sourced by density inhomogeneities and by fluctuations in the radiation from galaxies until the completion of reionization at redshift ~6. If the 21cm fluctuation power spectrum were isotropic, then the contributions of the various fluctuation mechanisms would combine and the interpretation of the total would be model dependent and subject to degeneracies. We show that peculiar velocities increase the power spectrum by a factor of ~2 compared to density fluctuations alone, and introduce an angular dependence in Fourier space. The resulting angular structure relative to the line of sight facilitates a simple separation of the power spectrum into several components, permitting an unambiguous determination of the primordial power spectrum of density fluctuations, and of the detailed properties of all other sources of 21cm fluctuations.Keywords
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