SIMBAD references

We present Keck NIRSPEC and Keck NIRES spectroscopy of sixteen metal-poor galaxies that have pre-existing optical observations. The near-infrared (NIR) spectroscopy specifically targets the He I λ10830 Å emission line, due to its sensitivity to the physical conditions of the gas in H II regions. We use these NIR observations, combined with optical spectroscopy, to determine the helium abundance of sixteen galaxies across a metallicity range 12+log₁₀(O/H) = 7.13-8.00. This data set is combined with two other samples where metallicity and helium abundance measurements can be secured: star-forming galaxies selected from the Sloan Digital Sky Survey spectroscopic database, and existing low-metallicity systems in the literature. We calculate a linear fit to these measurements, accounting for intrinsic scatter, and report a new determination of the primordial helium number abundance, y_P=0.0805_–0.0017^+0.0017, which corresponds to a primordial helium mass fraction Y_P=0.2436_–0.0040^+0.0039. Using our determination of the primordial helium abundance in combination with the latest primordial deuterium measurement, (D/H)_P×10⁵=2.527±0.030, we place a bound on the baryon density Ω_bh²=0.0215_–0.0005^+0.0005 and the effective number of neutrino species N_eff=2.85_–0.25^+0.28. These values are in 1.3σ agreement with those deduced from the Planck satellite observations of the temperature fluctuations imprinted on the cosmic microwave background.