The spin temperature of neutral hydrogen during cosmic pre-reionization
Author: Nusser, Adi
Source: Monthly Notices of the Royal Astronomical Society, Volume 359, Number 1, May 2005 , pp. 183-190(8)
Publisher: Blackwell Publishing
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Abstract:
We re-examine the role of collisions in decoupling the H i21-cm spin temperature from the cosmic microwave background (CMB). The cross-section for de-exciting the 21-cm transition in collisions with free electrons is more than 10 times larger than it is in collisions with other atoms. If the fraction of free electrons in the diffuse cosmic gas is between 10 and 30 per cent then collisions alone can decouple the spin temperature from the CMB, even in moderately underdense regions at z≳ 15 . This decoupling is especially important during the very early stages of reionization when a Lyα continuum background had yet to be established. As a detailed example, we develop a semi-analytic model to quantify 21-cm emission signatures from a diffuse gas which is partially ionized at z≳ 10 by an X-ray background. We find 21-cm differential brightness temperature fluctuations with a mean of ≳8 mK and an rms value as large as 5 mK, for a frequency resolution bandwidth of 100 kHz and a beamsize of 3 arcmin. Another example where free electron–atom collisions are important is during the recombination of bubbles ionized by short-lived UV sources. When the ionized fraction in these bubbles drops to 10–20 per cent their differential temperature can be as high as 10 mK.Keywords: intergalactic medium; cosmology: theory; large-scale structure of Universe
Document Type: Research article
DOI: 10.1111/j.1365-2966.2005.08894.x
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