TITLE:
The spin-resolved atomic velocity distribution and 21-cm line profile of dark-age gas.
AUTHOR(S):
Christopher M. Hirata and Kris Sigurdson (IAS).
DATE:
2006 May 02 (arXiv, v1, posted); 2006 May 03 (MNRAS, submitted);
2006 Nov 15 (revised); 2006 Nov 20 (MNRAS, accepted);
2007 Feb 02 (MNRAS, published).
AVAILABILITY:
arXiv astro-ph/0605071 (free);
Blackwell Synergy.
PUBLICATION INFORMATION: Mon. Not. R. Astron. Soc., 375, 1241--1264, 2007.
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" Ts
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, Ts=Ts(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.
ADS BIBLIOGRAPHIC CODE: 2007MNRAS.375.1241H
COMMENTS: N/A.