EMPRESS. XV. A New Determination of the Primordial Helium Abundance Suggesting a Moderately Low Y P Value

Abstract We present a new constraint on the primordial helium abundance, Y P , based on Subaru observations. A major source of uncertainty in previous Y P determinations is the lack of extremely metal-poor galaxies (EMPGs; 0.01–0.1 Z ⊙ ), which have metallicities a few to ten times lower than the metal-poor galaxies (MPGs; 0.1–0.4 Z ⊙ ) 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 i 10830 Å 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 P = 0.240 2 − 0.0040 + 0.0040 . This Y P value is ∼1 σ lower than most of the previous estimates, but agrees with recent determinations using EMPGs and the cosmic microwave background constraint from the Atacama Cosmology Telescope experiment. Our result indicates N eff = 2.5 4 − 0.25 + 0.20 , showing a mild (∼2 σ ) tension with the Standard Model and Planck results. These tensions may suggest a nonzero lepton asymmetry ( ξ e ≠ 0), which would alleviate the tension with ξ e = 0.0 5 − 0.03 + 0.02 . More observations of EMPGs and further assessments of systematic uncertainties are essential to test the potential tension more rigorously.