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 HeI 10830A emission line, due to its sensitivity to the physical conditions of the gas in HII regions. We use these NIR observations, combined with optical spectroscopy, to determine the helium abundance of sixteen galaxies across a metallicity range 12 + log (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, which corresponds to a primordial helium mass fraction Y_P = 0.2436+/-0.0040. Using our determination of the primordial helium abundance in combination with the latest primordial deuterium measurement, (D/H)_P x 10^5 =2.527+/-0.030, we place a bound on the baryon density Omega_bh^2 = 0.0215+/-0.0005 and the effective number of neutrino species Neff = 2.85+/-0.28. These values are in 1.3-sigma agreement with those deduced from the Planck satellite observations of the temperature fluctuations imprinted on the Cosmic Microwave Background.
Towards a measurement of the primordial helium isotope ratio
Durham University（杜伦大学）；Carnegie Science Observatories（卡内基科学天文台）；Institut d’Astrophysique de Paris, UMR 7095, CNRS and SU（巴黎天体物理学研究所，UMR 7095，法国国家科学研究中心和索邦大学）；University of Cambridge（剑桥大学）；European Southern Observatory（欧洲南方天文台）；Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseill（艾克斯-马赛大学，法国国家科学研究中心，马赛天体物理学实验室）
EMPRESS. XV. A New Determination of the Primordial Helium Abundance Suggesting a Moderately Low $Y_\mathrm{P}$ Value
Institute for Cosmic Ray Research, The University of Tokyo（东京大学宇宙射线研究所）；Department of Physics, Graduate School of Science, The University of Tokyo（东京大学理学研究科物理学系）；National Astronomical Observatory of Japan, National Institutes of Natural Sciences（日本国立天文台，国立自然科学研究院）；Department of Astronomical Science, SOKENDAI (The Graduate University for Advanced Studies)（SOKENDAI（ advanced studies研究生院）天体科学系）；Kavli Institute for the Physics and Mathematics of the Universe (WPI), University of Tokyo（东京大学 Kavli宇宙物理数学研究所 (WPI)）；Department of Physics, Tohoku University（东北大学物理学系）
