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

Kavli Affiliate: Masahiro Kawasaki

| First 5 Authors: Akinori Matsumoto, Masami Ouchi, Kimihiko Nakajima, Masahiro Kawasaki, Kai Murai

| Summary:

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$lambda$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
$Y_mathrm{P}=0.2379^{+0.0031}_{-0.0030}$ 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 ($sim 1sigma$) smaller due to our EMPGs. Including 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
$eta$ showing $gtrsim 1-2sigma$ tensions with the Standard Model and
citet{planck+18}. Motivated by the tensions, we allow the degeneracy parameter
of electron-neutrino $xi_e$ to vary as well as $N_mathrm{eff}$ and $eta$. We
obtain $xi_e = 0.05^{+0.03}_{-0.02}$, $N_mathrm{eff}=3.11^{+0.34}_{-0.31}$,
and $etatimes10^{10}=6.08^{+0.06}_{-0.06}$ from the $Y_mathrm{P}$ and
$D_mathrm{P}$ measurements with a prior of $eta$ 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 $1sigma$ level, which
could mitigate the Hubble tension.

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