The Spin-Resolved Atomic Velocity Distribution and 21-cm Line Profile of Dark-Age Gas

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dc.creatorHirata, Christopher M.
dc.creatorSigurdson, Kris
dc.date2006-05-02
dc.date.accessioned2026-07-07T10:23:04Z
dc.date.available2026-07-07T10:23:04Z
dc.descriptionThe 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'' T_s 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, T_s=T_s(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.
dc.description25 pages, 6 figures, submitted to Mon. Not. Roy. Astron. Soc
dc.identifierhttps://arxiv.org/abs/astro-ph/0605071
dc.identifierhttp://arxiv.org/abs/astro-ph/0605071
dc.identifierMon.Not.Roy.Astron.Soc.375:1241-1264,2007
dc.identifierdoi:10.1111/j.1365-2966.2006.11321.x
dc.identifier.urihttp://salesiana.dossiersoluciones.com/handle/123456789/175695
dc.subjectAstrophysics
dc.titleThe Spin-Resolved Atomic Velocity Distribution and 21-cm Line Profile of Dark-Age Gas
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