The LBT Y$_\mathrm{p}$ Project IV: A New Value of the Primordial Helium Abundance

TL;DR

This study delivers the most precise H II-region-based determination of the primordial helium abundance to date by analyzing 54 metal-poor regions with uniform, high-SNR LBT data and a rigorously updated physical model. Key advances include updated He/H emissivities from delz2022, updated H emissivities from stor2015, and the radiative-transfer corrections from kuri2025, alongside additional Paschen lines and refined line treatments. Through stringent χ^2 cuts and systematic flagging, 41 targets form the robust final dataset, including 15 ultra-low-metallicity objects enabling a weighted-average determination of the primordial helium mass fraction: $Y_p = 0.2458 \pm 0.0013$, i.e., 0.5% precision. This value is in strong agreement with the Planck/BBN expectation of $Y_p \approx 0.2467$, reinforcing concordance between early-Universe physics and CMB-derived constraints, while illustrating the power of high-quality, low-metallicity data for fundamental cosmology.

Abstract

We present a new determination of the primordial helium abundance based on new, high-quality Large Binocular Telescope (LBT) observations of 54 metal-poor H II regions. These regions have been observed and analyzed uniformly. We also describe a number of updates to our methodology, including updated helium emissivities. Enabled by the large, high-quality dataset, we examine our sample targets for potential systematic errors, which could bias their results. We perform a standard 95% confidence level $χ^2$ cut and find that a significantly larger fraction (47/54 = 87%) of our sample qualifies than for previous datasets. We also screen for quality and reliability, flagging targets which may introduce significant systematic errors, producing a dataset of 41 targets. In a significant breakthrough for the field, that dataset includes 15 high SNR targets with low metallicity (O/H < 4 $\times$ 10$^{-5}$). Due to this low-metallicity dataset, for the first time, a weighted average for determining the primordial helium abundance (Y$_\mathrm{p}$) is well-justified and produces a robust result. By weighted average of our 15 low-metallicity targets, we determine Y$_\mathrm{p}$ = 0.2458 $\pm$ 0.0013. This result achieves an unprecedented precision of 0.5%, and it is in good agreement with the BBN result, Y$_\mathrm{p}$ = 0.2467 $\pm$ 0.0002, based on the Planck determination of the baryon density.

Figures (21)

• Figure 1: The delz2022 emissivities plotted relative to those of port2012port2013 as a function of temperature at $\mathrm{n_{e}} = 100~\mathrm{cm}^{-3}$ for the core He emission lines, He I$\lambda$$\lambda$3889, 4026, 4471, 5876, 6678, 7065, 10830. At low density, all of the He emission lines employed in this work show changes of $\lesssim 1\%$, and typically a slight decrease, except for He I$\lambda$6678 which shows a more significant decrease.
• Figure 2: The delz2022 emissivities plotted relative to those of port2012port2013 as a function of density at $\mathrm{T_{e}} = 15,000\,\mathrm{K}$ for the core He emission lines, He I$\lambda$$\lambda$3889, 4026, 4471, 5876, 6678, 7065, 10830. Across the range of densities spanning our qualifying sample, all of the He emission lines employed in this work show changes of $\lesssim 1\%$, and typically a slight decrease, except for He I$\lambda$6678 which shows a more significant decrease.
• Figure 3: The kuri2025 radiative transfer corrections, $f_\tau$, plotted as a function of optical depth ($\tau = \tau_{{3889}}$) at $\mathrm{n_{e}} = 100~\mathrm{cm}^{-3}$ and $\mathrm{T_{e}} = 15,000\,\mathrm{K}$ for the He triplet emission lines, He I$\lambda$$\lambda$3889, 4026, 4471, 5876, 7065, 10830. As shown, radiative transfer results in a net decrease in He I$\lambda$3889 emission and a net increase in He I$\lambda$7065 emission, with those two lines by far the most strongly affected.
• Figure 4: The kuri2025 and benj2002 radiative transfer corrections, $f_\tau$, plotted as a function of optical depth ($\tau = \tau_{{3889}}$) at $\mathrm{n_{e}} = 100~\mathrm{cm}^{-3}$ and $\mathrm{T_{e}} = 15,000\,\mathrm{K}$ for He I$\lambda$3889 and He I$\lambda$7065, the two He emission lines most strongly affected by radiative transfer. The plot compares the limited-range fitting function and the trilinear interpolation program from benj2002 and the updated results of kuri2025. As is the case for all He triplet lines affected by radiative transfer, the magnitude of the radiative transfer effect is overestimated by extrapolation based on the limited-range fitting function, which was fit for $0 < \tau < 2$. The benj2002 interpolation and kuri2025 fit equations are more similar, though with differences that increase for higher optical depths (and also for higher densities, especially for He I$\lambda$7065).
• Figure 5: The collisional excitation relative to recombination rate for the hydrogen emission lines due to neutral hydrogen. The behavior is dominantly exponential with temperature, with, as expected, larger corrections for the lower level (lower energy) emission lines. Reproduced from aver2021.