Distinctive 21-cm structures of the first stars, galaxies and quasars

Hidenobu Yajima, Yuexing Li

Research output: Contribution to journalArticlepeer-review

11 Citations (Scopus)


Observations of the redshifted 21-cm line with forthcoming 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 Lyα photons. We find that Pop III stars and quasars produce a smooth transition from an ionized and hot state to a neutral and cold state, because of their hard spectral energy distribution with abundant ionizing photons, in contrast to the sharp transition in galaxies. Furthermore, Lyα scattering plays a dominant role in producing the 21-cm signal because 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 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. However, 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 travelling time of photons. We predict that future surveys from large radio arrays, such as the Murchison Widefield Array, the Low Frequency Array and the Square Kilometre Array, might be able to detect the 21-cm signals of primordial galaxies and quasars, but possibly not those of Pop III stars, because of their small angular diameters.

Original languageEnglish
Pages (from-to)3674-3684
Number of pages11
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
Publication statusPublished - 2014 Sep 29


  • Diffuse radiation
  • Galaxies: evolution
  • Galaxies: formation
  • Galaxies: high-redshift
  • Radiative transfer
  • Stars: Population III

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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