EMPRESS. XV. A New Determination of the Primordial Helium Abundance Suggesting a Moderately Low $Y_\mathrm{P}$ Value
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2025
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| author | Yanagisawa, Hiroto Ouchi, Masami Matsumoto, Akinori Kawasaki, Masahiro Murai, Kai Nakajima, Kimihiko Kohri, Kazunori Sugahara, Yuma Nagamine, Kentaro Tanaka, Ichi Kim, Ji Hoon Ono, Yoshiaki Nakane, Minami Fukushima, Keita Harikane, Yuichi Hirai, Yutaka Isobe, Yuki Kusakabe, Haruka Onodera, Masato Rauch, Michael Yajima, Hidenobu |
| author_facet | Yanagisawa, Hiroto Ouchi, Masami Matsumoto, Akinori Kawasaki, Masahiro Murai, Kai Nakajima, Kimihiko Kohri, Kazunori Sugahara, Yuma Nagamine, Kentaro Tanaka, Ichi Kim, Ji Hoon Ono, Yoshiaki Nakane, Minami Fukushima, Keita Harikane, Yuichi Hirai, Yutaka Isobe, Yuki Kusakabe, Haruka Onodera, Masato Rauch, Michael Yajima, Hidenobu |
| contents | We present a new constraint on the primordial helium abundance, $Y_\mathrm{P}$, based on Subaru observations. A major source of uncertainty in previous $Y_\mathrm{P}$ determinations is the lack of extremely metal-poor galaxies (EMPGs; $0.01-0.1\,Z_\odot$), which have metallicities a few to ten times lower than the metal-poor galaxies (MPGs; $0.1-0.4\,Z_\odot$) 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 \textsc{i} 10830Å\, 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 $Y_\mathrm{P} = 0.2402^{+0.0040}_{-0.0040}$. This $Y_\mathrm{P}$ value is $\sim1σ$ lower than most of the previous estimates, but agrees with recent determinations using EMPGs and the CMB constraint from the Atacama Cosmology Telescope (ACT) experiment. Our result indicates $N_\mathrm{eff} = 2.54^{+0.20}_{-0.25}$, showing a mild ($\sim2σ$) tension with the Standard Model and Planck results. These tensions may suggest a nonzero lepton asymmetry $(ξ_\mathrm{e}\neq0)$, which would alleviate the tension with $ξ_\mathrm{e} = 0.05^{+0.02}_{-0.03}$. More observations of EMPGs and further assessments of systematic uncertainties are essential to test the potential tension more rigorously. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2506_24050 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | EMPRESS. XV. A New Determination of the Primordial Helium Abundance Suggesting a Moderately Low $Y_\mathrm{P}$ Value Yanagisawa, Hiroto Ouchi, Masami Matsumoto, Akinori Kawasaki, Masahiro Murai, Kai Nakajima, Kimihiko Kohri, Kazunori Sugahara, Yuma Nagamine, Kentaro Tanaka, Ichi Kim, Ji Hoon Ono, Yoshiaki Nakane, Minami Fukushima, Keita Harikane, Yuichi Hirai, Yutaka Isobe, Yuki Kusakabe, Haruka Onodera, Masato Rauch, Michael Yajima, Hidenobu Astrophysics of Galaxies Cosmology and Nongalactic Astrophysics We present a new constraint on the primordial helium abundance, $Y_\mathrm{P}$, based on Subaru observations. A major source of uncertainty in previous $Y_\mathrm{P}$ determinations is the lack of extremely metal-poor galaxies (EMPGs; $0.01-0.1\,Z_\odot$), which have metallicities a few to ten times lower than the metal-poor galaxies (MPGs; $0.1-0.4\,Z_\odot$) 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 \textsc{i} 10830Å\, 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 $Y_\mathrm{P} = 0.2402^{+0.0040}_{-0.0040}$. This $Y_\mathrm{P}$ value is $\sim1σ$ lower than most of the previous estimates, but agrees with recent determinations using EMPGs and the CMB constraint from the Atacama Cosmology Telescope (ACT) experiment. Our result indicates $N_\mathrm{eff} = 2.54^{+0.20}_{-0.25}$, showing a mild ($\sim2σ$) tension with the Standard Model and Planck results. These tensions may suggest a nonzero lepton asymmetry $(ξ_\mathrm{e}\neq0)$, which would alleviate the tension with $ξ_\mathrm{e} = 0.05^{+0.02}_{-0.03}$. More observations of EMPGs and further assessments of systematic uncertainties are essential to test the potential tension more rigorously. |
| title | EMPRESS. XV. A New Determination of the Primordial Helium Abundance Suggesting a Moderately Low $Y_\mathrm{P}$ Value |
| topic | Astrophysics of Galaxies Cosmology and Nongalactic Astrophysics |
| url | https://arxiv.org/abs/2506.24050 |