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

Abstract

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 ($<0.1Z_{\odot }$) galaxies (EMPGs) having reliable He/H measurements with HeI$\hat{I}»$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 ($0.1-0.4Z_{\odot }$) galaxies with reliable He/H estimates, we obtain $Y_{\mathrm{P}}=0.2370_{-0.0033}^{+0.0034}$ by linear regression in the $(He/H)-(O/H)$ 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 ($\sim 1\ddot{I}ƒ$) 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 $\hat{I}\cdot$ showing $\gtrsim 1-2\ddot{I}ƒ$ tensions with the Standard Model and Planck Collaboration et al. (2020). Motivated by the tensions, we allow the degeneracy parameter of electron-neutrino $\hat{I}{¾}_e$ to vary as well as $N_{\mathrm{eff}}$ and $\hat{I}\cdot$. We obtain $\hat{I}{¾}_e=0.05_{-0.02}^{+0.03}$, $N_{\mathrm{eff}}=3.11_{-0.31}^{+0.34}$, and $\hat{I}\cdot \times 10^{10}=6.08_{-0.06}^{+0.06}$ from the $Y_{\mathrm{P}}$ and $D_{\mathrm{P}}$ measurements with a prior of $\hat{I}\cdot$ 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 $1\ddot{I}ƒ$ level, which could mitigate the Hubble tension.

22 pages, 9 figures, accepted for publication in ApJ