The PHLEK Survey : a new determination of the primordial helium abundance.

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 A 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 log O H + 10( ) = 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 0.0805 P 0.0017 0.0017, which corresponds to a primordial helium mass fraction = - + YP 0.2436 0.0040 0.0039. Using our determination of the primordial helium abundance in combination with the latest primordial deuterium measurement, D H 10 2.527 0.030 P ´=  5 ( ) , we place a bound on the baryon density W = - + bh 0.0215 2 0.0005 0.0005 and the effective number of neutrino species = - + Neff 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.