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}\(is best determined by observations of metal-poor galaxies, while there are only a few known local extremely metal-poor (\)<0.1 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.

Abstract

22 pages, 9 figures, accepted for publication in ApJ