Results from EDGES High-Band. III. New Constraints on Parameters of the Early Universe

We present new constraints on parameters of cosmic dawn and the epoch of reionization derived from the EDGES High-Band spectrum (90-190 MHz). The parameters are probed by evaluating global 21 cm signals generated with the recently developed Global21cm tool. This tool uses neural networks trained and tested on ∼30,000 spectra produced with semi-numerical simulations that assume the standard thermal evolution of the cosmic microwave background and the intergalactic medium. From our analysis, we constrain at 68% (1) the minimum virial circular velocity of star-forming halos to V _c <19.3 km s^-1, (2) the X-ray heating efficiency of early sources to f _X > 0.0042, and (3) the low-energy cutoff of the X-ray spectral energy distribution to ν _min <2.3 keV. We also constrain the star formation efficiency (f _∗), the electron scattering optical depth (τ _e), and the mean-free path of ionizing photons (R _mfp). We recompute the constraints after incorporating into the analysis four estimates for the neutral hydrogen fraction from high-z quasars and galaxies, and a prior on τ _e from Planck 2018. The largest impact of the external observations is on the parameters that most directly characterize reionization. Specifically, we derive the combined 68% constraints τ _e <0.063 and R _mfp > 27.5 Mpc. The external observations also have a significant effect on V _c due to its degeneracy with τ _e, while the constraints on f _∗, f _X, and ν _min, remain primarily determined by EDGES.