Distinctive 21 cm structures of the first stars, galaxies, and quasars
  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, Population (Pop) III stars, the first galaxies, and
the first 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 Pop III stars, the 21 cm signal
shows both emission and absorption regions around a small HII bubble. For
quasars, a large emission region in the 21 cm signal is produced, and the
absorption region decreases as the size of the HII bubble becomes large due to
the limited traveling time of photons. 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 not likely Pop III stars due to
its small angular diameter.
