The Spin-Resolved Atomic Velocity Distribution and 21-cm Line Profile of Dark-Age Gas

Abstract

The 21-cm hyperfine line of atomic hydrogen (HI) is a promising probe of the cosmic dark ages. In past treatments of 21-cm radiation it was assumed the hyperfine level populations of HI could be characterized by a velocity-independent ``spin temperature'' T_s determined by a competition between 21-cm radiative transitions, spin-changing collisions, and (at lower redshifts) Lyman-alpha scattering. However we show here that, if the collisional time is comparable to the radiative time, the spin temperature will depend on atomic velocity, T_s=T_s(v), and one must replace the usual hyperfine level rate equations with a Boltzmann equation describing the spin and velocity dependence of the HI distribution function. We construct here the Boltzmann equation relevant to the cosmic dark ages and solve it using a basis-function method. Accounting for the actual spin-resolved atomic velocity distribution results in up to a 2 per cent suppression of the 21-cm emissivity, and a redshift and angular-projection dependent suppression or enhancement of the linear power spectrum of 21-cm fluctuations of up to 5 per cent. The effect on the 21-cm line profile is more dramatic -- its full-width at half maximum (FWHM) can be enhanced by up to 60 per cent relative to the velocity-independent calculation. We discuss the implications for 21-cm tomography of the dark ages.

25 pages, 6 figures, submitted to Mon. Not. Roy. Astron. Soc

References in corpus (5)

• The Global 21 Centimeter Background from High Redshifts
• The CMBR fluctuations from HI perturbations prior to reionization
• Nonlinear clustering during the cosmic Dark Ages and its effect on the 21-cm background from minihalos
• Redshifted 21 Centimeter Emission from Minihalos Before Reionization
• Measuring the Primordial Deuterium Abundance During the Cosmic Dark Ages

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• The 21cm angular-power spectrum from the dark ages
• 21 cm signal from cosmic dawn: Imprints of spin temperature fluctuations and peculiar velocities
• Cosmic Reionization and the 21-cm signal: Comparison between an analytical model and a simulation
• The subtlety of Ly-a photons: changing the expected range of the 21-cm signal
• Spin Exchange Rates in Electron-Hydrogen Collisions
• Forecasted 21 cm constraints on compensated isocurvature perturbations
• Spin Exchange Rates in Proton-Hydrogen Collisions
• Background reionization history from omniscopes
• Particle decay in the early universe: predictions for 21 cm
• Cosmic 21-cm Fluctuations as a Probe of Fundamental Physics
• Intergalactic heating by Lyman-alpha photons including hyperfine structure corrections
• Primordial Magnetic Fields in Light of Dark Ages Global 21-cm Signal
• The Fundamentals of the 21-cm Line