EMPRESS. VIII. A New Determination of Primordial He Abundance with Extremely Metal-Poor Galaxies: A Suggestion of the Lepton Asymmetry and Implications for the Hubble Tension
Creators: name: Matsumoto, Akinori
name: Ouchi, Masami
name: Nakajima, Kimihiko
name: Kawasaki, Masahiro
name: Murai, Kai
name: Motohara, Kentaro
name: Harikane, Yuichi
name: Ono, Yoshiaki
name: Kushibiki, Kosuke
name: Koyama, Shuhei
name: Aoyama, Shohei
name: Konishi, Masahiro
name: Takahashi, Hidenori
name: Isobe, Yuki
name: Umeda, Hiroya
name: Sugahara, Yuma
name: Onodera, Masato
name: Nagamine, Kentaro
name: Kusakabe, Haruka
name: Hirai, Yutaka
name: Moriya, Takashi J.
name: Shibuya, Takatoshi
name: Komiyama, Yutaka
name: Fukushima, Keita
name: Fujimoto, Seiji
name: Hattori, Takashi
name: Hayashi, Kohei
name: Inoue, Akio K.
name: Kikuchihara, Shotaro
name: Kojima, Takashi
name: Koyama, Yusei
name: Lee, Chien-Hsiu
name: Mawatari, Ken
name: Miyata, Takashi
name: Nagao, Tohru
name: Ozaki, Shinobu
name: Rauch, Michael
name: Saito, Tomoki
name: Suzuki, Akihiro
name: Takeuchi, Tsutomu T.
name: Umemura, Masayuki
name: Xu, Yi
name: Yabe, Kiyoto
name: Zhang, Yechi
name: Yoshii, Yuzuru
Publisher: name: arXiv
Year: 2022
Subjects: subject: Cosmology and Nongalactic Astrophysics (astro-ph.CO)
subject: Astrophysics of Galaxies (astro-ph.GA)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: The primordial He abundance $Y_\mathrm{P}$ is a powerful probe of cosmology. Currently, $Y_\mathrm{P}$ is best determined by observations of metal-poor galaxies, while there are only a few known local extremely metal-poor ($&lt;0.1 Z_\odot$) galaxies (EMPGs) having reliable He/H measurements with HeI$λ$10830 near-infrared (NIR) emission. Here we present deep Subaru NIR spectroscopy for 10 EMPGs. Combining the existing optical data, He/H values of 5 out of the 10 EMPGs are reliably derived by the Markov chain Monte Carlo algorithm. Adding the existing 3 EMPGs and 51 moderately metal-poor ($0.1-0.4 Z_\odot$) galaxies with reliable He/H estimates, we obtain $Y_\mathrm{P}=0.2370^{+0.0034}_{-0.0033}$ by linear regression in the $\mathrm{(He/H)}-\mathrm{(O/H)}$ plane, where we increase the number of EMPGs from 3 to 8 anchoring He/H of the most metal-poor gas in galaxies. Although our $Y_\mathrm{P}$ measurement and previous measurements are consistent, our result is slightly ($\sim 1σ$) smaller due to our EMPGs. With our $Y_\mathrm{P}$ and the existing primordial deuterium $D_\mathrm{P}$ measurement, we constrain the effective number of neutrino species $N_\mathrm{eff}$ and the baryon-to-photon ratio $η$ showing $\gtrsim 1-2σ$ tensions with the Standard Model and Planck Collaboration et al. (2020). Motivated by the tensions, we allow the degeneracy parameter of electron-neutrino $ξ_e$ to vary as well as $N_\mathrm{eff}$ and $η$. We obtain $ξ_e = 0.05^{+0.03}_{-0.02}$, $N_\mathrm{eff}=3.11^{+0.34}_{-0.31}$, and $η\times10^{10}=6.08^{+0.06}_{-0.06}$ from the $Y_\mathrm{P}$ and $D_\mathrm{P}$ measurements with a prior of $η$ taken from Planck Collaboration et al. (2020). Our constraints suggest a lepton asymmetry and allow for a high value of $N_\mathrm{eff}$ within the $1σ$ level, which could mitigate the Hubble tension.
description: 22 pages, 9 figures, accepted for publication in ApJ