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Revised Primordial Helium Abundance Based on New Atomic Data

M. Peimbert, V. Luridiana, A. Peimbert

TL;DR

Using new atomic data for He I recombination and H I Balmer collisional excitation, the authors derive a refined primordial helium abundance of $Y_p = 0.2477 \pm 0.0029$, higher than their previous estimate. They implement tailored photoionization models and an extrapolation in metallicity via $\Delta Y/\Delta O = 3.3 \pm 0.7$ to remove heavy-element contamination, and they account for temperature structure with $t^2\neq0$. The key contributions include updated collisional rates, a self-consistent radiative cascade, and a revised error budget that identifies Balmer-line collisions as the dominant uncertainty. The resulting $Y_p$ is in good agreement with standard Big Bang nucleosynthesis predictions and WMAP-derived baryon density, reinforcing the concordance between helium, deuterium, and cosmic microwave background constraints and suggesting non-negligible temperature fluctuations in H II regions.

Abstract

We have derived a primordial helium abundance of Yp = 0.2477 +- 0.0029, based on new atomic physics computations of the recombination coefficients of He I and of the collisional excitation of the H I Balmer lines together with observations and photoionization models of metal-poor extragalactic H II regions. The new atomic data increase our previous determination of Yp by 0.0086, a very significant amount. By combining our Yp result with the predictions made by the standard Big Bang nucleosynthesis model, we find a baryon-to-photon ratio, η, in excellent agreement both with the ηvalue derived by the primordial deuterium abundance value observed in damped Lyman-αsystems and with the one obtained from the WMAP observations.

Revised Primordial Helium Abundance Based on New Atomic Data

TL;DR

Using new atomic data for He I recombination and H I Balmer collisional excitation, the authors derive a refined primordial helium abundance of , higher than their previous estimate. They implement tailored photoionization models and an extrapolation in metallicity via to remove heavy-element contamination, and they account for temperature structure with . The key contributions include updated collisional rates, a self-consistent radiative cascade, and a revised error budget that identifies Balmer-line collisions as the dominant uncertainty. The resulting is in good agreement with standard Big Bang nucleosynthesis predictions and WMAP-derived baryon density, reinforcing the concordance between helium, deuterium, and cosmic microwave background constraints and suggesting non-negligible temperature fluctuations in H II regions.

Abstract

We have derived a primordial helium abundance of Yp = 0.2477 +- 0.0029, based on new atomic physics computations of the recombination coefficients of He I and of the collisional excitation of the H I Balmer lines together with observations and photoionization models of metal-poor extragalactic H II regions. The new atomic data increase our previous determination of Yp by 0.0086, a very significant amount. By combining our Yp result with the predictions made by the standard Big Bang nucleosynthesis model, we find a baryon-to-photon ratio, η, in excellent agreement both with the ηvalue derived by the primordial deuterium abundance value observed in damped Lyman-αsystems and with the one obtained from the WMAP observations.

Paper Structure

This paper contains 15 sections, 7 equations, 1 figure.

Table of Contents

  1. Introduction
  2. Collisional enhancement of hydrogen Balmer lines
  3. Updated collisional rates
  4. Improved radiative cascade
  5. Model fitting
  6. Resulting estimates of collisional contribution to the Balmer lines
  7. Abundance determinations
  8. Adopted line intensities
  9. The $Y$ determinations
  10. The $Y_p$ determination
  11. The error budget
  12. Comparison with other $Y_p$ determinations
  13. Cosmological implications
  14. Discussion
  15. Conclusions

Figures (1)

  • Figure 1: Normalized contribution per unit radius to the emission in the main [O II] and [O III] lines and in $\rm{H}\beta_{col}$ in a model for NGC 346, showing that $\rm{H}\beta_{col}$ preferentially forms in the [O II] zone. The values are normalized to the maximum value in each line. The three panels correspond to different integrations, simulating a beam, a narrow slit, and a slit covering the whole object respectively.