We report the first upper limits on the power spectrum of 21 cm fluctuations during the Epoch of Reionization and Cosmic Dawn from Phase II of the Hydrogen Epoch of Reionization Array (HERA) experiment. HERA Phase II constitutes several significant improvements in the signal chain compared to Phase I, most notably resulting in expanded frequency bandwidth, from 50–250 MHz. In these first upper limits, we investigate a small two-week subset of the available Phase II observations, with a focus on identifying new systematic characteristics of the instrument, and establishing an analysis pipeline to account for them. We report 2σ upper limits in eight spectral bands, spanning 5.6 ≤ z ≤ 24.4 that are consistent with thermal noise at the 2σ level for k ≳ 0.6–0.9 h Mpc−1 (band dependent). Our tightest limit during Cosmic Dawn (z > 12) is 1.13 × 106 mK2 at (k = 0.55 h Mpc−1, z = 16.78), and during the EoR (5.5 < z < 12), it is 1.78 × 103 mK2 at (k = 0.70 h Mpc−1, z = 7.05). We find that mutual coupling has become our dominant systematic, leaking foreground power that strongly contaminates the low-k modes, resulting in the loss of modes from k = 0.35 to 0.55 compared to Phase I data.
First Results from HERA Phase II
Creators
- Abdurashidova, Zuhra1
- Adams, Tyrone2
-
Aguirre, James E.3
- Baartman, Rushelle2
-
Barkana, Rennan4
-
Berkhout, Lindsay M.5, 6
-
Bernardi, Gianni2, 7, 8
- Billings, Tashalee S.3
-
Bizarria, Bruno B.9, 10
-
Bowman, Judd D.5
-
Breitman, Daniela11
-
Bull, Philip9, 12
-
Burba, Jacob9
-
Byrne, Ruby13
- Carey, Steven14
-
Chandra, Rajorshi Sushovan15
-
Chen, Kai-Feng16
-
Choudhuri, Samir17
-
Cox, Tyler1
-
DeBoer, David R.1
-
de Lera Acedo, Eloy14
- Dexter, Matt1
-
Dhandha, Jiten14
-
Dillon, Joshua S.1
- Dynes, Scott16
- Eksteen, Nico2
- Ely, John14
-
Ewall-Wice, Aaron1
- Fagnoni, Nicolas14
-
Fialkov, Anastasia14
-
Furlanetto, Steven R.18
- Gale-Sides, Kingsley14
-
Garsden, Hugh9
-
Gorce, Adelie19
-
Gorthi, Deepthi1
- Halday, Ziyaad2
-
Hazelton, Bryna J.13
-
Hewitt, Jacqueline N.16
-
Hickish, Jack1
- Huang, Tian14
-
Jacobs, Daniel C.5
-
Josaitis, Alec14
-
Kern, Nicholas S.16, 20
-
Kerrigan, Joshua21
-
Kittiwisit, Piyanat2, 12
-
Kolopanis, Matthew5
-
Lanman, Adam21
-
La Plante, Paul22, 23
-
Liu, Adrian1, 6
-
Ma, Yin-Zhe24
-
MacMahon, David H. E.1
- Malan, Lourence2
- Malgas, Cresshim2
- Malgas, Keith2
- Marero, Bradley2
- Martinot, Zachary E.3
-
McBride, Lisa6, 19
-
Mesinger, Andrei11, 25
-
Mirocha, Jordan26, 27
- Mohamed-Hinds, Nicel13
- Molewa, Mathakane2
-
Morales, Miguel F.13
-
Muñoz, Julian B.28
-
Murray, Steven G.11
- Nikolic, Bojan14
- Nuwegeld, Hans2
-
Parsons, Aaron R.1
-
Pascua, Robert1, 6, 29, 30
-
Patra, Nipanjana1, 31
-
Pochinda, Simon14
-
Qin, Yuxiang32
-
Rath, Eleanor16
- Razavi-Ghods, Nima14
- Riley, Daniel16
- Rosie, Kathryn2
-
Santos, Mario G.2, 12
-
Singh, Saurabh15
-
Storer, Dara13
- Swarts, Hilton2
-
Tan, Jianrong3
-
Thélie, Emilie28
- van Wyngaarden, Pieter2
-
Wilensky, Michael J.6, 33
-
Williams, Peter K. G.34, 35
- Zheng, Haoxuan16
- (The HERA Collaboration)
-
1.
University of California, Berkeley
-
2.
South African Radio Astronomy Observatory
-
3.
University of Pennsylvania
-
4.
Tel Aviv University
-
5.
Arizona State University
-
6.
McGill University
-
7.
Istituto di Radioastronomia di Bologna
-
8.
Rhodes University
-
9.
University of Manchester
-
10.
National Institute for Space Research
-
11.
Scuola Normale Superiore di Pisa
-
12.
University of the Western Cape
-
13.
University of Washington
-
14.
University of Cambridge
-
15.
Raman Research Institute
-
16.
Massachusetts Institute of Technology
-
17.
Indian Institute of Technology Madras
-
18.
University of California, Los Angeles
-
19.
Institut d'Astrophysique Spatiale
-
20.
University of Michigan–Ann Arbor
-
21.
Brown University
- 22. Department of Computer Science, University of Nevada, Las Vegas, NV 89154, USA
- 23. Nevada Center for Astrophysics, University of Nevada, Las Vegas, NV 89154, USA
-
24.
Stellenbosch University
-
25.
University of Catania
-
26.
Jet Propulsion Lab
-
27.
California Institute of Technology
-
28.
The University of Texas at Austin
-
29.
University of Toronto
-
30.
Perimeter Institute
-
31.
International Centre for Radio Astronomy Research
-
32.
Australian National University
- 33. CITA National Fellow
-
34.
Harvard-Smithsonian Center for Astrophysics
-
35.
American Astronomical Society
Abstract
Copyright and License
© 2026. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Acknowledgement
This material is based upon work supported by the National Science Foundation under grant Nos. 1636646, 1836019, 1352519, and 1407804 as well as institutional support from the HERA collaboration partners. This research is funded in part by the Gordon and Betty Moore Foundation through grant GBMF5212 to the Massachusetts Institute of Technology. HERA is hosted by the South African Radio Astronomy Observatory, which is a facility of the National Research Foundation, an agency of the Department of Science and Innovation.
This work used Bridges-2 at Pittsburgh Supercomputing Center through allocation PHY201142 from the Advanced Cyberinfrastructure Coordination Ecosystem: Services & Support (ACCESS) program, which is supported by U.S. National Science Foundation grant Nos. 2138259, 2138286, 2138307, 2137603, and 2138296. We acknowledge the use of the Ilifu cloud computing facility (www.ilifu.ac.za) and the support from the Inter-University Institute for Data Intensive Astronomy (IDIA; https://www.idia.ac.za).
S.G.M. has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No. 101067043. This result is part of a project that has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant agreement No. 948764; P.B., J.B., M.J.W.). B.B.B. acknowledges the funding received from FAPESP under process 2024/12902-3. N.S.K. acknowledges support from NASA through the NASA Hubble Fellowship grant No. HST-HF2-51533.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. J.M. was supported by an appointment to the NASA Postdoctoral Program at the Jet Propulsion Laboratory/California Institute of Technology, administered by Oak Ridge Associated Universities under contract with NASA. A.M. acknowledges support from the Italian Ministry of Universities and Research (MUR) through the PRIN project “Optimal inference from radio images of the epoch of reionization,” and the PNRR project “Centro Nazionale di Ricerca in High Performance Computing, Big Data e Quantum Computing.” A.C.L. acknowledges support from the Natural Sciences and Engineering Research Council of Canada through their Discovery Grants Program, as well as the William Dawson Scholar program at McGill University. R.P. acknowledges support from the Faculty of Arts & Science at the University of Toronto and the Dunlap Institute. The Dunlap Institute is funded through an endowment established by the David Dunlap family and the University of Toronto.
Facilities
HERA - .
Software References
numpy (C. R. Harris et al. 2020), scipy (P. Virtanen et al. 2020), matplotlib (J. D. Hunter 2007), astropy (Astropy Collaboration et al. 2013, 2018), jupyter (T. Kluyver et al. 2016), pyuvdata (B. J. Hazelton et al. 2017; G. Keating et al. 2025), fftvis (T. A. Cox et al. 2025). hera_opm (P. L. Plante et al. 2021), hera_cal (https://github.com/hera-team/hera_cal, hera_pspec (https://github.com/hera-team/hera_pspec
Files
Abdurashidova_2026_ApJ_998_33.pdf
Additional details
Related works
- Is new version of
- Discussion Paper: arXiv:2511.21289 (arXiv)
Funding
- National Science Foundation
- 1636646
- National Science Foundation
- 1836019
- National Science Foundation
- 1352519
- National Science Foundation
- 1407804
- Gordon and Betty Moore Foundation
- GBMF5212
- European Union
- Marie Skłodowska-Curie 101067043
- European Research Council
- 948764
- Fundação de Amparo à Pesquisa do Estado de São Paulo
- 2024/12902-3
- National Aeronautics and Space Administration
- HST-HF2-51533.001
- National Aeronautics and Space Administration
- NAS5-26555
- Ministero dell'università e della ricerca
- PRIN -
- Natural Sciences and Engineering Research Council
- McGill University
- University of Toronto
Dates
- Submitted
-
2025-08-19
- Accepted
-
2025-12-14
- Available
-
2026-02-02Published
Caltech Custom Metadata
- Caltech groups
- Division of Physics, Mathematics and Astronomy (PMA)
- Publication Status
- Published