Hydrogen 2p–2s transition: Signals from the Epochs of Recombination and Reionization

We propose a method to study the epoch of reionization based on the possible observation of 2p-2s fine-structure lines from the neutral hydrogen outside the cosmological H II regions enveloping QSOs and other ionizing sources in the reionization era. We show that for parameters typical of luminous sources observed at z ≃ 6.3, the strength of this signal, which is proportional to the H I fraction, has a brightness temperature ≃ 20 μK for a fully neutral medium. The fine-structure line from this redshift is observable at ν ≃ 1 GHz and we discuss prospects for the detection with several operational and future radio telescopes. We also compute the characteristics of this signal from the epoch of recombination; the peak brightness is expected to be ≃ 100 μK and this signal appears in the frequency range 5-10 MHz. The signal from the recombination era is nearly impossible to detect owing to the extreme brightness of the Galactic emission at these frequencies