First Results on the Epoch of Reionization from First Light with SARAS 2

Saurabh Singh, Ravi Subrahmanyan, N. Udaya Shankar, Mayuri Sathyanarayana Rao, Anastasia Fialkov, Aviad Cohen, Rennan Barkana, B. S. Girish, A. Raghunathan, R. Somashekar, K. S. Srivani

Research output: Contribution to journalArticlepeer-review


Long-wavelength spectral distortions in the cosmic microwave background arising from the 21 cm transition in neutral hydrogen are a key probe of the Cosmic Dawn and the Epoch of Reionization. These features may reveal the nature of the first stars and ultra-faint galaxies that transformed the spin temperature and ionization state of the primordial gas. SARAS2 is a spectral radiometer purposely designed for the precision measurement of these monopole or all-sky global 21 cm spectral distortions. We use 63hr nighttime observations of the radio background in the frequency band 110-200MHz, with the radiometer deployed at the Timbaktu Collective in Southern India, to derive likelihoods for plausible redshifted 21 cm signals predicted by theoretical models. First light with SARAS 2 disfavors the class of models that feature weak X-ray heating (with fX ≤ 0.1) and rapid reionization (with peak dTb/dzb ≥ 120 mK per unit redshift interval).

Original languageEnglish
Article numberL12
JournalAstrophysical Journal Letters
Issue number2
StatePublished - 20 Aug 2017


  • cosmic background radiation
  • cosmology: observations
  • dark ages
  • first stars-methods: observational
  • reionization


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