The effects of He I λ10830 on helium abundance determinations

Abstract

Observations of helium and hydrogen emission lines from metal-poor\nextragalactic H II regions provide an independent method for determining the\nprimordial helium abundance, Y_p. Traditionally, the emission lines employed\nare in the visible wavelength range, and the number of suitable lines is\nlimited. Furthermore, when using these lines, large systematic uncertainties in\nhelium abundance determinations arise due to the degeneracy of physical\nparameters, such as temperature and density. Recently, Izotov, Thuan, & Guseva\n(2014) have pioneered adding the He 10830 infrared emission line in helium\nabundance determinations. The strong electron density dependence of He 10830\nmakes it ideal for better constraining density, potentially breaking the\ndegeneracy with temperature. We revisit our analysis of the dataset published\nby Izotov, Thuan, & Stasinska (2007) and incorporate the newly available\nobservations of He 10830 by scaling them using the observed-to-theoretical\nPaschen-gamma ratio. The solutions are better constrained, in particular for\nelectron density, temperature, and the neutral hydrogen fraction, improving the\nmodel fit to data, with the result that more spectra now pass screening for\nquality and reliability, in addition to a standard 95% confidence level cut.\nFurthermore, the addition of He 10830 decreases the uncertainty on the helium\nabundance for all galaxies, with reductions in the uncertainty ranging from\n10-80%. Overall, we find a reduction in the uncertainty on Y_p by over 50%.\nFrom a regression to zero metallicity, we determine Y_p = 0.2449 +/- 0.0040,\nconsistent with the BBN result, Y_p = 0.2470 +/- 0.0002, based on the Planck\ndetermination of the baryon density. The dramatic improvement in the\nuncertainty from incorporating He 10830 strongly supports the case for\nsimultaneous (thus not requiring scaling) observations of visible and infrared\nhelium emission line spectra.\n