{
  "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.",
  "abstract_shown": true,
  "affiliations": [
    "The University of Tokyo",
    "National Institutes of Natural Sciences",
    "SOKENDAI (The Graduate University for Advanced Studies)",
    "University of Tokyo",
    "Tohoku University",
    "Kanazawa University",
    "National Astronomical Observatory of Japan",
    "Graduate University for AdvancedStudies (SOKENDAI)",
    "Theory Center",
    "Waseda University",
    "Osaka University",
    "University of Nevada",
    "Subaru Telescope",
    "Seoul National University",
    "Tohoku University of Community Service and Science",
    "University of Cambridge",
    "The Observatories of the Carnegie Institution for Science",
    "University of Tsukuba"
  ],
  "article_type": null,
  "arxiv": null,
  "author_total": 21,
  "authors": [
    {
      "affs": [
        0
      ],
      "name": "Hiroto Yanagisawa",
      "orcid": "0009-0006-6763-4245"
    },
    {
      "affs": [
        1,
        2,
        0,
        3
      ],
      "name": "Masami Ouchi",
      "orcid": "0000-0002-1049-6658"
    },
    {
      "affs": [
        0
      ],
      "name": "Akinori Matsumoto"
    },
    {
      "affs": [
        0
      ],
      "name": "Masahiro Kawasaki",
      "orcid": "0000-0001-9991-7051"
    },
    {
      "affs": [
        4
      ],
      "name": "Kai Murai",
      "orcid": "0000-0003-2879-1724"
    },
    {
      "affs": [
        5,
        6
      ],
      "name": "Kimihiko Nakajima",
      "orcid": "0000-0003-2965-5070"
    },
    {
      "affs": [
        7,
        1,
        8,
        3
      ],
      "name": "Kazunori Kohri",
      "orcid": "0000-0003-3764-8612"
    },
    {
      "affs": [
        9
      ],
      "name": "Yuma Sugahara",
      "orcid": "0000-0001-6958-7856"
    },
    {
      "affs": [
        10,
        11,
        3
      ],
      "name": "Kentaro Nagamine",
      "orcid": "0000-0001-7457-8487"
    },
    {
      "affs": [
        12
      ],
      "name": "Ichi Tanaka",
      "orcid": "0000-0002-4937-4738"
    },
    {
      "affs": [
        13
      ],
      "name": "Jinwoo Kim",
      "orcid": "0000-0003-1603-2675"
    },
    {
      "affs": [
        0
      ],
      "name": "Yoshiaki Ono",
      "orcid": "0000-0001-9011-7605"
    },
    {
      "affs": [
        0
      ],
      "name": "Minami Nakane",
      "orcid": "0009-0000-1999-5472"
    },
    {
      "affs": [
        13
      ],
      "name": "Keita Fukushima",
      "orcid": "0000-0002-5045-6052"
    },
    {
      "affs": [
        0
      ],
      "name": "Yuichi Harikane",
      "orcid": "0000-0002-6047-430X"
    },
    {
      "affs": [
        14
      ],
      "name": "Yutaka Hirai",
      "orcid": "0000-0002-5661-033X"
    },
    {
      "affs": [
        15,
        9
      ],
      "name": "Yuki Isobe",
      "orcid": "0000-0001-7730-8634"
    },
    {
      "affs": [
        0
      ],
      "name": "Haruka Kusakabe",
      "orcid": "0000-0002-3801-434X"
    },
    {
      "affs": [
        2,
        12
      ],
      "name": "Masato Onodera",
      "orcid": "0000-0003-3228-7264"
    },
    {
      "affs": [
        16
      ],
      "name": "Michael Rauch",
      "orcid": "0000-0002-1690-3488"
    },
    {
      "affs": [
        17
      ],
      "name": "Hidenobu Yajima",
      "orcid": "0000-0002-1319-3433"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/dzcgwwjtdtz59.bib",
  "cited_by": 8,
  "doi": "10.3847/1538-4357/ae64fc",
  "fulltext_url": null,
  "id": "dzcgwwjtdtz59",
  "issns": [
    "0004-637X",
    "1538-4357"
  ],
  "landing_url": "https://doi.org/10.3847/1538-4357/ae64fc",
  "language": "en",
  "licence": "CC-BY",
  "licence_label": "CC BY",
  "licence_source": "journal",
  "licence_url": "https://creativecommons.org/licenses/by/4.0/",
  "metadata_licence": null,
  "native_subjects": [
    [
      "openalex",
      "3103|Galaxies: Formation, Evolution, Phenomena"
    ]
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  "openalex": "W7163583873",
  "pdf_url": null,
  "pmcid": null,
  "pmid": null,
  "published": "2026-06-04",
  "publisher": "IOP Publishing",
  "ris": "https://research.draconitic.com/paper/dzcgwwjtdtz59.ris",
  "sources": [
    {
      "first_seen": "2026-11-04T13:21:02Z",
      "native_id": "W7163583873",
      "source": "openalex",
      "url": "https://doi.org/10.3847/1538-4357/ae64fc"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "openalex:3103|Galaxies: Formation, Evolution, Phenomena",
      "subfield": "galaxies"
    }
  ],
  "title": "EMPRESS. XV. A New Determination of the Primordial Helium Abundance Suggesting a Moderately Low Y P Value",
  "title_text": "EMPRESS. XV. A New Determination of the Primordial Helium Abundance Suggesting a Moderately Low Y P Value",
  "type": "peer-reviewed",
  "updated_at": "2026-11-06T13:20:25Z",
  "url": "https://research.draconitic.com/paper/dzcgwwjtdtz59",
  "venue": "The Astrophysical Journal",
  "versions": []
}