The primordial abundance of 4He: a self-consistent empirical analysis of systematic effects in a large sample of low-metallicity HII regions

TL;DR

This paper delivers a precise determination of the primordial helium mass fraction $Y_p$ by analyzing a large, homogeneous set of low-metallicity H II region spectra and a secondary SDSS-based sample. It implements a self-consistent Monte Carlo framework to account eight systematic effects affecting He and H line intensities, including He I emissivities, reddening, temperature structure, absorption, hydrogen collisional excitation, ionization structure, and deviations from case B. Depending on the He I emissivities used, the inferred $Y_p$ is either $0.2472 \pm 0.0012$ (older emissivities) or $0.2516 \pm 0.0011$ (new Porter et al. emissivities), with the former agreeing with SBBN predictions and the latter implying a small deviation that may point to new physics. The results constrain the baryon density $\\Omega_b h^2$ and the effective number of neutrino species $N_\nu$, showing tension with SBBN only if the newer emissivities are correct, thus underscoring the crucial role of atomic data in cosmological inferences. The work demonstrates that a rigorous treatment of systematics, combined with a large dataset, can achieve the precision required for meaningful cosmological tests from primordial element abundances.

Abstract

We determine the primordial helium mass fraction Yp using 93 spectra of 86 low-metallicity extragalactic HII regions. This sample constitutes the largest and most homogeneous high-quality data sets in existence for the determination of Yp. For comparison and to improve the statistics in our investigation of systematic effects affecting the Yp determination, we have also considered a sample of 271 low-metallicity HII regions selected from the DR5 of the SDSS. Although this larger sample shows more scatter, it gives results that are consistent at the 2sigma level with our original sample. We have considered known systematic effects which may affect the 4He abundance determination. They include different sets of HeI line emissivities and reddening laws, collisional and fluorescent enhancements of HeI recombination lines, underlying HeI stellar absorption lines, collisional excitation of hydrogen lines, temperature and ionization structure of the HII region, and deviation of HeI and H emission line intensities from case B. However, the most likely value of Yp depends on the adopted set of HeI line emissivities. Using Monte Carlo methods to solve simultaneously the above systematic effects we find a primordial helium mass fraction Yp = 0.2472+/-0.0012 when using the HeI emissivities from Benjamin et al. (1999, 2002) and 0.2516+/-0.0011 when using those from Porter et al. (2005). The first value agrees well with the value given by SBBN theory, while the value obtained with likely more accurate emissivities of Porter et al. (2005) is higher at the 2sigma level. This latter value, if confirmed, would imply slight deviations from SBBN.