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 \lambda 10830Å 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 + \textnormal { log } _ { 10 } ( 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 ^ { +0.0039 } _ { -0.0040 } .
Using our determination of the primordial helium abundance in combination with the latest primordial deuterium measurement , ( D / H ) _ { P } \times 10 ^ { 5 } = 2.527 \pm 0.030 , we place a bound on the baryon density \Omega _ { b } h ^ { 2 } = 0.0215 ^ { +0.0005 } _ { -0.0005 } and the effective number of neutrino species N _ { eff } = 2.85 ^ { +0.28 } _ { -0.25 } .
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 .