HERA Phase I Limits on the Cosmic 21 cm Signal: Constraints on Astrophysics and Cosmology during the Epoch of Reionization
Authors: Abdurashidova Zara , Aguirre James E. , Alexander Paul , Ali Zaki S. , Balfour Yanga , Barkana Rennan , Beardsley Adam P. , Bernardi Gianni , Billings Tashalee S. , Bowman Judd D. , Bradley Richard F. , Bull Philip , Burba Jacob , Carey Steve , Carilli Chris L. , Cheng Carina , DeBoer David R. , Dexter Matt , de Lera Acedo Eloy , Dillon Joshua S. , Ely John , Ewall-Wice Aaron , Fagnoni Nicolas , Fialkov Anastasia , Fritz Randall , Furlanetto Steven R. , Gale-Sides Kingsley , Glendenning Brian , Gorthi Deepthi , Greig Bradley , Grobbelaar Jasper , Halday Ziyaad , Hazelton Bryna J. , Heimersheim Stefan , Hewitt Jacqueline N. , Hickish Jack , Jacobs Daniel C. , Julius Austin , Kern Nicholas S. , Kerrigan Joshua , Kittiwisit Piyanat , Kohn Saul A. , Kolopanis Matthew , Lanman Adam , La Plante Paul , Lekalake Telalo , Lewis David , Liu Adrian , Ma Yin-Zhe , MacMahon David , Malan Lourence , Malgas Cresshim , Maree Matthys , Martinot Zachary E. , Matsetela Eunice , Mesinger Andrei , Mirocha Jordan , Molewa Mathakane , Morales Miguel F. , Mosiane Tshegofalang , Muñoz Julian B. , Murray Steven G. , Neben Abraham R. , Nikolic Bojan , Nunhokee Chuneeta D. , Parsons Aaron R. , Patra Nipanjana , Pieterse Samantha , Pober Jonathan C. , Qin Yuxiang , Razavi-Ghods Nima , Reis Itamar , Ringuette Jon , Robnett James , Rosie Kathryn , Santos Mario G. , Sikder Sudipta , Sims Peter , Smith Craig , Syce Angelo , Thyagarajan Nithyanandan , Williams Peter K. G. , Zheng Haoxuan 
Published: January 2022

Abstract
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 ∼ 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 ∼ 8, independent of uncertainties about IGM ionization and the radio background. Combining HERA limits with complementary 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 a lower bound on X-ray heating, a previously unconstrained aspects of early galaxies. For example, if the cosmic microwave background dominates the z ∼ 8 radio background, the new HERA limits imply that the first galaxies produced X-rays more efficiently than local ones. The z ∼ 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,ν
/SFR > 4 × 1024 W Hz−1

M

⊙

−
1

yr and L

X
/SFR < 7.6 × 1039 erg s−1

M

⊙

−
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.