The primordial helium abundance from updated emissivities

TL;DR

This work updates the primordial helium abundance determination by integrating the PFSD He I emissivities into a previous AOS3 framework, reanalyzing the HeBCD ITS07 dataset with a rigorous 95% CL screening. The updated emissivities raise line fluxes and reduce $y^{+}$ for a fixed flux, leading to a revised $Y_p = 0.2465 \pm 0.0097$, in good agreement with Planck's $Y_p = 0.2485 \pm 0.0002$. The analysis confirms the robustness of a Monte Carlo approach but reveals larger object-by-object uncertainties and a somewhat increased sample variance, underscoring the need for higher-quality spectra to tighten the primordial helium abundance constraints. The results support the Planck-derived baryon density while emphasizing ongoing challenges in modeling and measurement that limit precision.

Abstract

Observations of metal-poor extragalactic H II regions allow the determination of the primordial helium abundance, Y_p. The He I emissivities are the foundation of the model of the H II region's emission. Porter, Ferland, Storey, & Detisch (2012) have recently published updated He I emissivities based on improved photoionization cross-sections. We incorporate these new atomic data and update our recent Markov Chain Monte Carlo analysis of the dataset published by Izotov, Thuan, & Stasinska (2007). As before, cuts are made to promote quality and reliability, and only solutions which fit the data within 95% confidence level are used to determine the primordial He abundance. The previously qualifying dataset is almost entirely retained and with strong concordance between the physical parameters. Overall, an upward bias from the new emissivities leads to a decrease in Y_p. In addition, we find a general trend to larger uncertainties in individual objects (due to changes in the emissivities) and an increased variance (due to additional objects included). From a regression to zero metallicity, we determine Y_p = 0.2465 +/- 0.0097, in good agreement with the Planck result of Y_p = 0.2485 +/- 0.0002. In the future, a better understanding of why a large fraction of spectra are not well fit by the model will be crucial to achieving an increase in the precision of the primordial helium abundance determination.

Figures (5)

• Figure 1: Comparison of the PFSD emissivities, $\frac{{\tilde{E}}(\lambda)}{E(H\beta)}$, to those of PFM, including the collisional correction, $\frac{E(\lambda)}{E(H\beta)}(1+\frac{C}{R}(\lambda))$ (both for n$_e$ = 100 cm$^{-3}$). The PFM fits are the dashed lines while the PFSD fits are solid. The progression is, left to right, top to bottom, by wavelength: $\lambda\lambda$3889, 4026, 4471, 5876, 6678, 7065.
• Figure 2: The PFSD emissivities, $\frac{{\tilde{E}}(\lambda)}{E(H\beta)}$, plotted relative to those of PFM, including the collisional correction, $\frac{E(\lambda)}{E(H\beta)}(1+\frac{C}{R}(\lambda))$ (both for n$_e$ = 100 cm$^{-3}$). The updated emissivities all show an increase compared to the older emissivities, but the relative shifts are clearly not the same for all six lines. He I $\lambda\lambda$4026, 4471, and 7065, all show similar, small increases of $<1\%$, but $\lambda\lambda$5876 and 6678 show significantly larger increases.
• Figure 3: Plot of the ratio of y$^{+}$ as determined in this work using the PFSD emissivities to that of AOS3 using the PFM emissivities. This ratio is plotted versus O/H for the 70 objects for which He I $\lambda$4026 was detected. There is a clear trend to lower abundance values with the new PFSD emissivities. Additionally, since metallicity is negatively correlated with temperature, the ratio tends further away from unity with decreasing abundance in accordance with the increasing divergence in the emissivity ratio as the temperature increases (see figure \ref{['EmissivityRelative']}).
• Figure 4: Plot of y$^{+}$ vs O/H for the 27 objects meeting the prescribed reliability standards. The upward triangles signify points flagged for large outlier values in optical depth or underlying absorption. The downward triangles signify points flagged for large neutral hydrogen fractions. The diamonds signify points flagged for both large outlier values in optical depth or underlying absorption and for large neutral hydrogen fractions.
• Figure 5: Helium abundance (mass fraction) versus oxygen to hydrogen ratio regression calculating the primordial helium abundance.