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Peer-reviewedCC BYThe Astrophysical Journal

EMPRESS. XV. A New Determination of the Primordial Helium Abundance Suggesting a Moderately Low Y P Value

  • Hiroto Yanagisawa1
  • Masami Ouchi2,3,1,4
  • Akinori Matsumoto1
  • Masahiro Kawasaki1
  • Kai Murai5
  • Kimihiko Nakajima6,7
  • Kazunori Kohri8,2,9,4
  • Yuma Sugahara10
  • Kentaro Nagamine11,12,4
  • Ichi Tanaka13
  • Jinwoo Kim14
  • Yoshiaki Ono1
9 more authors
  • Minami Nakane1
  • Keita Fukushima14
  • Yuichi Harikane1
  • Yutaka Hirai15
  • Yuki Isobe16,10
  • Haruka Kusakabe1
  • Masato Onodera3,13
  • Michael Rauch17
  • Hidenobu Yajima18
18 affiliations
  1. 1The University of Tokyo
  2. 2National Institutes of Natural Sciences
  3. 3SOKENDAI (The Graduate University for Advanced Studies)
  4. 4University of Tokyo
  5. 5Tohoku University
  6. 6Kanazawa University
  7. 7National Astronomical Observatory of Japan
  8. 8Graduate University for AdvancedStudies (SOKENDAI)
  9. 9Theory Center
  10. 10Waseda University
  11. 11Osaka University
  12. 12University of Nevada
  13. 13Subaru Telescope
  14. 14Seoul National University
  15. 15Tohoku University of Community Service and Science
  16. 16University of Cambridge
  17. 17The Observatories of the Carnegie Institution for Science
  18. 18University of Tsukuba
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Abstract

We present a new constraint on the primordial helium abundance, Y P , based on Subaru observations. A major source of uncertainty in previous Y P determinations is the lack of extremely metal-poor galaxies (EMPGs; 0.01–0.1 Z ⊙ ), which have metallicities a few to ten times lower than the metal-poor galaxies (MPGs; 0.1–0.4 Z ⊙ ) predominantly used in earlier studies, requiring substantial extrapolation to zero metallicity. Here, we perform Subaru near-infrared spectroscopy of 29 galaxies, including 14 EMPGs. By incorporating existing optical spectra, we derive He/H for each galaxy using photoionization modeling of helium and hydrogen emission lines, including the He i 10830 Å line to break the density–temperature degeneracy. After carefully selecting galaxies with robust He/H determinations, and adding 58 galaxies from previous studies, we obtain Y P = 0.240 2 − 0.0040 + 0.0040 . This Y P value is ∼1 σ lower than most of the previous estimates, but agrees with recent determinations using EMPGs and the cosmic microwave background constraint from the Atacama Cosmology Telescope experiment. Our result indicates N eff = 2.5 4 − 0.25 + 0.20 , showing a mild (∼2 σ ) tension with the Standard Model and Planck results. These tensions may suggest a nonzero lepton asymmetry ( ξ e ≠ 0), which would alleviate the tension with ξ e = 0.0 5 − 0.03 + 0.02 . More observations of EMPGs and further assessments of systematic uncertainties are essential to test the potential tension more rigorously.

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Cite

BibTeXRIS
BibTeX
@article{yanagisawa2026empress,
  title = {{EMPRESS. XV. A New Determination of the Primordial Helium Abundance Suggesting a Moderately Low Y P Value}},
  author = {Hiroto Yanagisawa and Masami Ouchi and Akinori Matsumoto and Masahiro Kawasaki and Kai Murai and Kimihiko Nakajima and Kazunori Kohri and Yuma Sugahara and Kentaro Nagamine and Ichi Tanaka and Jinwoo Kim and Yoshiaki Ono and Minami Nakane and Keita Fukushima and Yuichi Harikane and Yutaka Hirai and Yuki Isobe and Haruka Kusakabe and Masato Onodera and Michael Rauch and Hidenobu Yajima},
  journal = {The Astrophysical Journal},
  year = {2026},
  month = jun,
  doi = {10.3847/1538-4357/ae64fc},
  url = {https://doi.org/10.3847/1538-4357/ae64fc},
  note = {CC BY},
}
RIS
TY  - JOUR
TI  - EMPRESS. XV. A New Determination of the Primordial Helium Abundance Suggesting a Moderately Low Y P Value
AU  - Hiroto Yanagisawa
AU  - Masami Ouchi
AU  - Akinori Matsumoto
AU  - Masahiro Kawasaki
AU  - Kai Murai
AU  - Kimihiko Nakajima
AU  - Kazunori Kohri
AU  - Yuma Sugahara
AU  - Kentaro Nagamine
AU  - Ichi Tanaka
AU  - Jinwoo Kim
AU  - Yoshiaki Ono
AU  - Minami Nakane
AU  - Keita Fukushima
AU  - Yuichi Harikane
AU  - Yutaka Hirai
AU  - Yuki Isobe
AU  - Haruka Kusakabe
AU  - Masato Onodera
AU  - Michael Rauch
AU  - Hidenobu Yajima
PY  - 2026
DA  - 2026/06/04
JO  - The Astrophysical Journal
DO  - 10.3847/1538-4357/ae64fc
UR  - https://doi.org/10.3847/1538-4357/ae64fc
AB  - We present a new constraint on the primordial helium abundance, Y P , based on Subaru observations. A major source of uncertainty in previous Y P determinations is the lack of extremely metal-poor galaxies (EMPGs; 0.01–0.1 Z ⊙ ), which have metallicities a few to ten times lower than the metal-poor galaxies (MPGs; 0.1–0.4 Z ⊙ ) predominantly used in earlier studies, requiring substantial extrapolation to zero metallicity. Here, we perform Subaru near-infrared spectroscopy of 29 galaxies, including 14 EMPGs. By incorporating existing optical spectra, we derive He/H for each galaxy using photoionization modeling of helium and hydrogen emission lines, including the He i 10830 Å line to break the density–temperature degeneracy. After carefully selecting galaxies with robust He/H determinations, and adding 58 galaxies from previous studies, we obtain Y P = 0.240 2 − 0.0040 + 0.0040 . This Y P value is ∼1 σ lower than most of the previous estimates, but agrees with recent determinations using EMPGs and the cosmic microwave background constraint from the Atacama Cosmology Telescope experiment. Our result indicates N eff = 2.5 4 − 0.25 + 0.20 , showing a mild (∼2 σ ) tension with the Standard Model and Planck results. These tensions may suggest a nonzero lepton asymmetry ( ξ e ≠ 0), which would alleviate the tension with ξ e = 0.0 5 − 0.03 + 0.02 . More observations of EMPGs and further assessments of systematic uncertainties are essential to test the potential tension more rigorously.
SN  - 0004-637X
SN  - 1538-4357
ER  -