Monday, August 5, 2013

1308.0381 (Hidenobu Yajima et al.)

Distinctive 21 cm structures of the first stars, galaxies, and quasars    [PDF]

Hidenobu Yajima, Yuexing Li
Observations of the redshifted 21 cm line with upcoming radio telescopes promise to transform our understanding of the cosmic reionization. To unravel the underlying physical process, we investigate the 21 cm structures of three different ionizing sources, Pop III stars, the first galaxies and quasars, by using radiative transfer simulations that include both ionization of neutral hydrogen and resonant scattering of Lya photons. We find that Pop III stars and quasars produce a smooth transition from an ionized and hot state to a neutral and cold one, owing to their hard spectral energy distribution with abundant ionizing photons, in contrast to the sharp transition in galaxies. Furthermore, Lya scattering plays a dominant role in producing the 21 cm signal as it determines the relation between hydrogen spin temperature and gas kinetic temperature. This effect, also called Wouthuysen-Field coupling, depends strongly on the ionizing source. It is the strongest around galaxies, where the spin temperature is highly coupled to that of the gas, resulting in extended absorption troughs in the 21 cm brightness temperature. On the other hand, in the case of quasars, since Lya photons cannot propagate far from the source due to its large HII region and limited traveling time, only emission of the 21 cm is produced, while for Pop III stars, the 21 cm signal falls in between, showing both emission and absorption region around a small HII bubble. We predict that future surveys from large radio arrays such as MWA, LOFAR and SKA may be able to detect the 21 cm signals of primordial galaxies and quasars, but unlikely Pop III stars due to its small angular diameter.
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