Spin Exchange Rates in Electron-Hydrogen Collisions

Abstract

The spin temperature of neutral hydrogen, which determines the 21 cm optical depth and brightness temperature, is set by the competition between radiative and collisional processes. In the high-redshift intergalactic medium, the dominant collisions are typically those between hydrogen atoms. However, collisions with electrons couple much more efficiently to the spin state of hydrogen than do collisions with other hydrogen atoms and thus become important once the ionized fraction exceeds ~1%. Here we compute the rate at which electron-hydrogen collisions change the hydrogen spin. Previous calculations included only S-wave scattering and ignored resonances near the n=2 threshold. We provide accurate results, including all partial wave terms through the F-wave, for the de-excitation rate at temperatures T_K < 15,000 K; beyond that point, excitation to n>=2 hydrogen levels becomes significant. Accurate electron-hydrogen collision rates at higher temperatures are not necessary, because collisional excitation in this regime inevitably produces Lyman-alpha photons, which in turn dominate spin exchange when T_K > 6200 K even in the absence of radiative sources. Our rates differ from previous calculations by several percent over the temperature range of interest. We also consider some simple astrophysical examples where our spin de-excitation rates are useful.

submitted to MNRAS, 9 pages, 5 figures

References in corpus (9)

• Cosmology at Low Frequencies: The 21 cm Transition and the High-Redshift Universe
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• The CMBR fluctuations from HI perturbations prior to reionization
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• Nonlinear clustering during the cosmic Dark Ages and its effect on the 21-cm background from minihalos
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Cited by in corpus (4)

• 21 cm fluctuations from inhomogeneous X-ray heating before reionization
• Evolution of the 21 cm signal throughout cosmic history
• Cosmic Reionization and the 21-cm signal: Comparison between an analytical model and a simulation
• Spin Exchange Rates in Proton-Hydrogen Collisions