http://arxiv.org/abs/2203.09617
The primordial He abundance $Y_P$ is a powerful probe of cosmology. Currently, $Y_P$ 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 and He/H determinations for 10 EMPGs, combining the existing optical data and 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 $Y_P=0.2379^{+0.0031}{-0.0030}$ by linear regression in the (He/H)-(O/H) plane, where our observations increase the number of EMPGs from 3 to 13 anchoring He/H of the most metal-poor gas in galaxies. Although our $Y_P$ measurement and previous measurements are consistent, our result is slightly (~ 1$\sigma$) smaller due to our EMPGs. Including the existing primordial deuterium $D_P$ constraints, we estimate the effective number of neutrino species to be $N{eff}=2.41^{+0.19}{-0.21}$ showing a > 2 $\sigma$ tension with the Standard Model value ($N{eff}=3.046$), which may be a hint of an asymmetry in electron-neutrino $\nu_e$ and anti-electron neutrino $\bar{\nu}e$. Allowing the degeneracy parameter of electron-neutrino $\xi_e$ to vary as well as $N{eff}$ and the baryon-to-photon ratio $\eta$, we obtain $\xi_e$ = $0.05^{+0.03}{-0.03}$, $N{eff}=3.22^{+0.33}{-0.30}$, and $\eta\times10^{10}=6.13^{+0.04}{-0.04}$ from the $Y_P$ and $D_P$ measurements with a prior of $\eta$ taken from Planck Collaboration et al. (2020). Our constraints suggest a $\nu_e – \bar{\nu}e$ asymmetry and allow for a high value of $N{eff}$ within the 1$\sigma$ level, which could mitigate the Hubble tension.
A. Matsumoto, M. Ouchi, K. Nakajima, et. al.
Mon, 21 Mar 22
46/60
Comments: 19 pages, 7 figures, submitted to ApJ
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