The PHLEK Survey: A New Determination of the Primordial Helium Abundance

Abstract

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   λ 10830 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  = 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, , which corresponds to a primordial helium mass fraction . Using our determination of the primordial helium abundance in combination with the latest primordial deuterium measurement, , we place a bound on the baryon density and the effective number of neutrino species . These values are in 1.3 σ agreement with those deduced from the Planck satellite observations of the temperature fluctuations imprinted on the cosmic microwave background.