Kavli Affiliate: Masahiro Kawasaki
| First 5 Authors: Hiroto Yanagisawa, Hiroto Yanagisawa, , ,
| 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 AA 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 $sim1sigma$ 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 ($1-2sigma$) tension with the Standard Model and Planck
results. These tensions may suggest a nonzero lepton asymmetry, which would be
alleviated with $xi_mathrme = 0.05^+0.02_-0.02$. More observations of
EMPGs and further assessments of systematic uncertainties are essential to test
the potential tension more rigorously.
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