The 21-cm signature of early relic H II regions

We calculate the spin temperature and 21-cm brightness of early H II regions around the first stars. We use outputs from cosmological radiation-hydrodynamics simulations of the formation and evolution of early H II regions. In the pre-re-ionization era, H II regions around massive primordial stars have diameters of a few kpc. The gas within the H II regions is almost fully ionized, but begins recombining after the central stars die off. The relic H II regions are then seen as bright emission sources in hydrogen 21 cm. We make brightness temperature maps of the H II regions, accounting for radiative coupling with Lyman alpha photons in a simplified manner. The spin temperature in the relic H II region is close to the gas kinetic temperature, generally several hundred to several thousand degrees. We show that the relic H II region can be as bright as delta T-b similar to 100mK in differential temperature against the cosmic microwave background for an angular resolution of subarcseconds. While individual early H II patches will not be identified by currently planned radio telescopes, the collective fluctuations from early H II regions might imprint signatures in the 21-cm background.