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Abstract: We show that, due to the high optical depth of the intergalactic medium toLyman-alpha photons before the Epoch of Reionization, the Lyman-alphascattering rate responsible for the Wouthuysen-Field effect from an isolatedsource will be negligible unless 1 there is sufficient time for thescattering photons to establish a steady state, or 2 the scattering gas isundergoing internal expansion or has a peculiar motion of tens to hundreds ofkm-s away from the source. We present steady-state solutions in the radiativediffusion approximation for the radiation field trapped in a clump of gas andshow that this may result in an enhancement, by a factor of up to 10^6, of thestrength of the Wouthuysen-Field effect over that obtained from thefree-streaming limit. Solutions to the time-dependent diffusion equation,however, suggest that the timescales required to reach such a steady state willgenerally exceed the source lifetimes. In the presence of internal expansion, asteady state may be established as photons are redshifted into the red wing,and significant enhancement in the scattering rate may again be produced.Alternatively, a substantial scattering rate may arise in systems with apeculiar motion away from the source that redshifts the received radiation intothe resonance line centre. As a consequence, at epochs z<30, when collisionaldecoupling is small except in dense regions, and prior to the establishment ofany large-scale diffuse radiation field of resonance line photons, the 21cmsignature from the Intergalactic Medium produced by the Wouthuysen-Field effectwill in general trace the peculiar velocity field of the gas in addition to itsdensity structure.

Autor: Jonathan Higgins University of Edinburgh, Avery Meiksin University of Edinburgh


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