HERA Phase I Limits on the Cosmic 21cm Signal: Constraints on Astrophysics and Cosmology During the Epoch of Reionization

Recently, the Hydrogen Epoch of Reionization Array (HERA) collaboration has produced the experiment’s first upper limits on the power spectrum of 21-cm fluctuations in the early Universe, with measurements at z ~ 8 and 10. Here, we use several independent theoretical models to infer constraints on the intergalactic medium (IGM) and galaxies during the epoch of reionization (EoR) from these limits. The new constraints require the IGM to have been heated above the adiabatic cooling threshold by z ~ 8, independent of uncertainties about the IGM ionization state and the nature of the radio background. Combining HERA limits with galaxy and EoR observations constrains the spin temperature of the z ~ 8 neutral IGM to 27 K < T_S < 630 K (2.3 K < T_S < 640 K) at 68% (95%) confidence. They therefore also place limits on previously unconstrained aspects of early galaxies, limiting models with low X-ray heating, especially in the presence of strong radio emission. For example, if the z ~ 8 radio background is dominated by the CMB, the new HERA limits imply that the first galaxies were more efficient in producing X-rays than local ones (with soft X-ray luminosities per star-formation rate constrained to L_X/SFR = {10^40.2, 10^41.9} erg/s/M_⊙ yr at 68% confidence), consistent with expectations of X-ray binaries in low-metallicity environments. The z ~ 10 limits require even earlier heating if dark-matter interactions (e.g., through millicharges) cool down the hydrogen gas. Using a model in which an extra radio background is produced by galaxies, we rule out (at 95% confidence) the combination of high radio and low X-ray luminosities of L_r,nu/SFR > 3.9 × 10²⁴ W/Hz/M_⊙ yr and L_X/SFR < 10⁴⁰ erg/s/M_⊙ yr. The new HERA upper limits neither support nor disfavor a cosmological interpretation of the recent EDGES detection. The analysis framework described here provides a foundation for the interpretation of future HERA results.