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

Recently, the Hydrogen Epoch of Reionization Array (HERA) has produced the experiment's first upper limits on the power spectrum of 21 cm fluctuations at z similar to 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 from these limits. We find that the IGM must have been heated above the adiabatic-cooling threshold by z similar to 8, independent of uncertainties about IGM ionization and the radio background. Combining HERA limits with complementary observations constrains the spin temperature of the z similar to 8 neutral IGM to 27 K <(T) over bar (S)> 630 K (2.3 K <(T) over bar (S)> 640 K) at 68% (95%) confidence. They therefore also place a lower bound on X-ray heating, a previously unconstrained aspects of early galaxies. For example, if the cosmic microwave background dominates the z similar to 8 radio background, the new HERA limits imply that the first galaxies produced X-rays more efficiently than local ones. The z similar to 10 limits require even earlier heating if dark-matter interactions cool the hydrogen gas. If 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,L-nu/SFR > 4 x 10(24) W Hz(-1) M-circle dot(-1) yr and L-X/SFR < 7.6 x 10(39) erg s(-1) M-circle dot(-1) yr. The new HERA upper limits neither support nor disfavor a cosmological interpretation of the recent Experiment to Detect the Global EOR Signature (EDGES) measurement. The framework described here provides a foundation for the interpretation of future HERA results.

Automated summary

Recent upper limits from the Hydrogen Epoch of Reionization Array (HERA) provide insights on the intergalactic medium (IGM) and galaxies during the epoch of reionization, setting lower bounds on X-ray heating efficiency of early galaxies. The constraints on neutral IGM spin temperature and implications for X-ray heating efficiency provide valuable information for future HERA results. These new limits neither confirm nor rule out the cosmological interpretation of recent measurements by the Experiment to Detect the Global EOR Signature (EDGES).