EMPRESS. XV. A New Determination of the Primordial Helium Abundance Suggesting a Moderately Low $Y_\mathrm{P}$ Value

Abstract

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.

Figures (9)

• Figure 1: Example NIR spectra of our sample galaxies, J2302+00049 taken with SWIMS and J0825+1846 taken with MOIRCS. The blue histograms and gray shaded regions represent the spectra and their error. The He i 10830 Å and P$\gamma$ emission lines are highlighted. The red lines indicate the best-fit double gaussian.
• Figure 2: Probability distribution functions (PDFs) of the model parameters for J0815+2156 obtained with YMCMC. The one and two dimensional PDFs are shown in the diagonal and off-diagonal panels, respectively. The contours denote the 1$\sigma$, 2$\sigma$, and 3$\sigma$ confidence levels. The blue solid and black dashed lines indicate the best-fit values and the 68% confidence levels, respectively.
• Figure 3: Comparison of the recovered and observed flux ratios of J0014-0043. The black dots represent the relative error of the recovered flux ratios compared to the observations, while the red error bars show the observational $2\sigma$ errors.
• Figure 4: Helium abundance as a function of oxygen abundance. The red and blue data points denote the EMPGs and MPGs, respectively. The squares, diamonds, and circles represent the galaxies presented in this study, Matsumoto+2022, and Hsyu+2020, respectively. The black line shows the best-fit linear model, while the dark and light gray shaded regions are the 1$\sigma$ and 3$\sigma$ errors of the linear fitting, respectively.
• Figure 5: Same as Figure \ref{['fig:linearfit']}, but the $y^+$ values of the galaxies presented in this study are derived with the prior distributions that allows negative values for $c(\mathrm{H}\beta),~a_\mathrm{H},~ a_\mathrm{He}$, and $\tau_\mathrm{He}$. The gray squares are the values derived with the prior distributions that do not allow these four parameters to be negative (i.e., the values shown in Figure \ref{['fig:linearfit']}). Note that the gray data points are not used in the linear regression.