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 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. We find that the IGM must 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 a lower bound on X-ray heating, a previously unconstrained aspects of early galaxies. For example, if the CMB dominates the z~8 radio background, the new HERA limits imply that the first galaxies produced X-rays more efficiently than local ones (with soft band X-ray luminosities per star formation rate constrained to L_X/SFR = { 10^40.2, 10^41.9 } erg/s/(M_sun/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 x 10^24 W/Hz/(M_sun/yr) and L_X/SFR<10^40 erg/s/(M_sun/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.
Reference:
HERA Phase I Limits on the Cosmic 21-cm Signal: Constraints on Astrophysics and Cosmology During the Epoch of Reionization, The HERA Collaboration: Zara Abdurashidova, James E. Aguirre, Paul Alexander, Zaki Ali, Yanga Balfour, Rennan Barkana, Adam Beardsley, Gianni Bernardi, Tashalee Billings, Judd Bowman, Richard Bradley, Phillip Bull, Jacob Burba, Steven Carey, Christopher Carilli, Carina Cheng, David DeBoer, Matthew Dexter, Eloy de Lera Acedo, Joshua Dillon, John Ely, Aaron Ewall-Wice, Nicolas Fagnoni, Anastasia Fialkov, Randall Fritz, Steven Furlanetto, Kingsley Gale-Sides, Brian Glendenning, Deepthi Gorthi, Bradley Greig, Jasper Grobbelaar, Ziyaad Halday, Bryna Hazelton, Stefan Heimersheim, Jacqueline Hewitt, Jack Hickish, Daniel Jacobs, Austin Julius, Nicholas Kern, Joshua Kerrigan, Piyanat Kittiwisit, Saul Kohn, Matthew Kolopanis, Adam Lanman, Paul La Plante, Telalo Lekalake, David Lewis, Adrian Liu, Yin-Zhe Ma, David MacMahon, Lourence Malan, Cresshim Malgas, Matthys Maree, Zachary Martinot, Eunice Matsetela, Andrei Mesinger, Jordan Mirocha, Mathakane Molewa, Miguel Morales, Tshegofalang Mosiane, Julian Munoz, Steven Murray, Abraham Neben, Bojan Nikolic, Chuneeta Devi Nunhokee, Aaron Parsons, Nipanjana Patra, Samantha Pieterse, Jonathan Pober, Yuxiang Qin, N. Razavi-Ghods, Itamar Reis, Jon Ringuette, James Robnett, Kathryn Rosie, Mario Santos, Sudipta Sikder, Peter Sims, Craig Smith, Angelo Syce, Nithyanandan Thyagarajan, Peter Williams, Haoxuan Zheng, submitted to ApJ, arXiv:2108.07282