Kavli Affiliate: Masahiro Kawasaki
| Summary:
We present a new constraint on the primordial helium abundance, $Y_mathrmP$, based on Subaru observations. A major source of uncertainty in previous $Y_mathrmP$ determinations is the lack of extremely metal-poor galaxies (EMPGs; $0.01-0.1Z_odot$), which have metallicities a few to ten times lower than the metal-poor galaxies (MPGs; $0.1-0.4Z_odot$) 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 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 $Y_mathrmP = 0.2387^+0.0036_-0.0031$. This $Y_mathrmP$ 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 $N_mathrmeff = 2.54^+0.21_-0.20$, showing a mild ($1-2σ$) tension with the Standard Model and Planck results. These tensions may suggest a nonzero lepton asymmetry, which would be alleviated with $ξ_mathrme = 0.05^+0.02_-0.02$. More observations of EMPGs and further assessments of systematic uncertainties are essential to test the potential tension more rigorously.
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