TITLE:
Wouthuysen-Field coupling strength and application to high-redshift 21 cm radiation.
AUTHOR(S):
Christopher M. Hirata (Princeton Univ.).
DATE:
2005 Jul 05 (arXiv, v1, posted); 2005 Jul 05 (MNRAS, submitted); 2005 Nov 26 (revised); 2005 Dec 05 (MNRAS, accepted); 2005 Dec 05 (arXiv, v2, posted);
2006 Jan 18 (MNRAS, published).
AVAILABILITY:
arXiv astro-ph/0507102 (free);
Blackwell-Synergy (requires subscription).
PUBLICATION INFORMATION: Mon. Not. R. Astron. Soc., 367, 259--274 (2006).
ABSTRACT:
The first ultraviolet sources in the universe are expected to have coupled
the HI spin temperature to the gas kinetic temperature via scattering in
the Lya resonance (the "Wouthuysen-Field effect"). By establishing an HI spin
temperature different from the temperature of the cosmic microwave
background, the Wouthuysen-Field effect should allow observations of HI during the
reionization epoch in the redshifted 21 cm hyperfine line. This paper
investigates four mechanisms that can affect the strength of the Wouthuysen-Field
effect that were not previously considered: (1) Photons redshifting into
the HI Lyman resonances may excite an H atom and result in a radiative
cascade terminating in two-photon 2s1/2-->1s1/2 emission,
rather than always degrading to Lya as usually assumed. (2) The fine
structure of the Lya resonance alters the photon frequency distribution
and leads to a suppression of the scattering rate. (3) The spin-flip
scatterings change the frequency of the photon and cause the photon
spectrum to relax not to the kinetic temperature of the gas but to a
temperature between the kinetic and spin temperatures, effectively
reducing the strength of the Wouthuysen-Field coupling. (4) Near line centre, a
photon can change its frequency by several times the line width in a
single scattering event, thus potentially invalidating the usual
calculation of the Lya spectral distortion based on the diffusion
approximation. It is shown that (1) suppresses the Wouthuysen-Field coupling strength
by a factor of up to ~2, while
(2) and (3) are important only at low kinetic temperatures.
Effect (4) has a <=3 per cent effect for
kinetic temperatures Tk>=2K. In
particular if the pre-reionization intergalactic medium was efficiently
heated by X-rays, only effect (1) is important. Fitting formulae for the
Wouthuysen-Field coupling strength are provided for the range of Tk>=2K and
Gunn-Peterson optical depth 105<t<107 so that
all of these effects can be easily incorporated into 21 cm codes.
ADS BIBLIOGRAPHIC CODE: 2006MNRAS.367..259H
COMMENTS: N/A.