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

Abstract

Abstract 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 (&lt;0.1 Z ⊙ ) galaxies (EMPGs) having reliable He/H measurements with He i λ 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 ⊙ ) galaxies with reliable He/H estimates, we obtain <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>Y</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">P</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>0.2370</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.0034</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.0033</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> by linear regression in the (He/H) − (O/H) plane, where we increase the number of EMPGs from three to eight 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 σ ) smaller due to our EMPGs. Including the existing primordial deuterium D P measurement, we constrain the effective number of neutrino species N eff and the baryon-to-photon ratio η showing ≳1–2 σ tensions with the Standard Model and Planck Collaboration et al. (2020). Motivated by the tensions, we allow the degeneracy parameter of the electron neutrino ξ e , as well as N eff and η , to vary. We obtain <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>ξ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>e</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>0.05</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.02</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.03</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>N</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>eff</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>3.11</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.31</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.34</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>η</mml:mi> <mml:mo>×</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> </mml:msup> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>6.08</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.06</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.06</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> from the Y P and D P measurements with a prior of η taken from Planck Collaboration et al. Our constraints suggest a lepton asymmetry and allow for a high value of N eff within the 1 σ level, which could mitigate the Hubble tension.