Currently, the primordial helium abundance is best estimated through spectroscopic observations of HII regions in metal-poor galaxies. However these determinations are limited by several systematic uncertainties which ultimately limit our ability to accurately ascertain the primordial abundance. In this study, we improve the methodologies for solving for the reddening, the emission contributions from collisional excitation of the HI atoms, the effects underlying absorption in the HI and He I emission lines, and the treatment of the blended HI and He I emission at λ3889 with the aim of lowering the systematic uncertainties in helium abundance determinations. To apply these methods, we have obtained observations of the He I λ10830 emission line in the brightest HII region in the extremely metal-poor (3% Z⊙) galaxy Leo P with the LUCI1 instrument on the LBT. We combine this measurement with previous MODS/LBT observations to derive an improved helium abundance. In doing so, our present analysis results in a decrease in the uncertainty in the helium abundance of Leo P by approximately 70%. This result is combined with data from other observations to estimate the primordial helium mass fraction, Yp = 0.2453 ± 0.0034.
Bibliographical noteFunding Information:
This paper uses data taken with the MODS spectrographs built with funding from NSF grant AST-9987045 and the NSF Telescope System Instrumentation Program (TSIP), with additional funds from the Ohio Board of Regents and the Ohio State University Office of Research. This paper made use of the modsIDL spectral data reduction pipeline developed in part with funds provided by NSF Grant AST-1108693. This work was based in part on observations made with the Large Binocular Telescope (LBT). The LBT is an international collaboration among institutions in the United States, Italy and Germany. The LBT Corporation partners are: the University of Arizona on behalf of the Arizona university system;
The authors owe special thanks to Connor Ballance and Martin O’Mullane for their effective collision strength calculations for electron impact excitation for neutral hydrogen. This work made use of v2.2 of the Binary Population and Spectral Synthesis (BPASS) models, as described in Eldridge, Stanway et al. (2017) and Stanway & Eldridge et al. (2018). We would like to express our gratitude to the referee for the very helpful and timely report. The work of KAO is supported in part by DOE grant DE-SC0011842. EDS is grateful for partial support from the University of Minnesota. EA benefited greatly from three visits during sabbatical to the University of Minnesota and is grateful to the University of Minnesota and the William I. Fine Theoretical Physics Institute for the support.
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