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Keywords:

  • atomic processes;
  • hydrodynamics;
  • ii regions;
  • cosmology: observations;
  • dark ages, reionization, first stars;
  • radio lines: general

ABSTRACT

We study the excitation of the fine structure levels of C i, C ii and O i by ultraviolet (UV) photons around strong UV sources which are also ionizing sources of the cosmological reionization at a redshift of ∼10. The evolutions of ionized regions around a point source are calculated by solving rate equations for non-equilibrium chemistry. Signals of UV photons through the fine structure lines are considered to be stronger at locations of more abundant chemical species of C i, C ii and O i. Such environments would be realized at locations where strong fluxes of non-ionizing UV line photons available for the pumping up of fine structure levels exist, and simultaneously ionizing UV photons are effectively shielded by dense H i regions. Signals from H i regions of moderately large densities induced by redshifted UV photons emitted at the point sources are found to be dominantly large over those of others. We discuss the detectability of the signals, and show that signals from idealized environments will be possibly detected by radio observations with next-generation arrays to come after the Atacama Large Millimeter/submillimeter Array (ALMA).