The PHLEK Survey : a new determination of the primordial helium abundance.
2020
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.
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