A new determination of the primordial He abundance using the HeI 10830A emission line: cosmological implications

TL;DR

This study introduces near-infrared spectroscopic observations of the density-sensitive He I 10830Å line for 45 low-metallicity H II regions to refine the primordial helium abundance. By jointly analyzing optical and NIR He lines and employing a Monte Carlo framework that models extinction, fluorescence, collisional processes, and ionisation structure, the authors derive a tighter Y–O/H relation and obtain Y$_p$ = 0.2551 ± 0.0022. The result is slightly higher than the standard BBN value, suggesting potential deviations in the early-Universe expansion rate and a nonzero effective number of neutrino species Neff ≈ 3.58 ± 0.50, consistent with a mild presence of dark radiation. Joint fitting with D/H supports a baryon density Ω_b h^2 ≈ 0.0240 ± 0.0034 and reinforces the potential need for nonstandard relativistic degrees of freedom. Overall, the He I 10830Å line markedly improves the precision and reliability of $Y_{ m p}$ determinations and their cosmological implications.

Abstract

We present near-infrared spectroscopic observations of the high-intensity HeI 10830 emission line in 45 low-metallicity HII regions. We combined these NIR data with spectroscopic data in the optical range to derive the primordial He abundance. The use of the HeI 10830A line, the intensity of which is very sensitive to the density of the HII region, greatly improves the determination of the physical conditions in the He^+ zone. This results in a considerably tighter Y - O/H linear regression compared to all previous studies. We extracted a final sample of 28 HII regions with Hbeta equivalent width EW(Hbeta)>150A, excitation parameter O^2+/O>0.8, and with helium mass fraction Y derived with an accuracy better than 3%. With this final sample we derived a primordial He mass fraction Yp = 0.2551+/-0.0022. The derived value of Yp is higher than the one predicted by the standard big bang nucleosynthesis (SBBN) model. Using our derived Yp together with D/H = (2.53+/-0.04)x10^-5, and the chi^2 technique, we found that the best agreement between these light element abundances is achieved in a cosmological model with a baryon mass density Omega_b h^2 = 0.0240+/-0.0017 (68% CL), +/-0.0028 (95.4% CL), +/-0.0034 (99% CL) and an effective number of neutrino species Neff = 3.58+/-0.25 (68% CL), +/-0.40 (95.4% CL), +/-0.50 (99% CL). A non-standard value of Neff is preferred at the 99% CL, implying the possible existence of additional types of neutrino species.

Figures (11)

• Figure 1: Representative APO spectra of H ii regions showing the He i$\lambda$10830Å and P$\gamma$10940Å emission lines. The spectrum in (a) is that of a high-density H ii region while the one in (b) is that of low-density H ii region.
• Figure 2: Redshift-corrected spectra of SBS 0335$-$052E in the optical range (blue) and in the near-infrared range (red) obtained with the 8.2m Very Large Telescope (VLT) and the 8.4m Large Binocular Telescope (LBT), respectively. The He i emission lines are marked by vertical dotted lines and labelled.
• Figure 3: Ratios of $Y$($\lambda$) derived from individual He i emission lines to the weighted mean helium abundance $Y_{\rm wm}$. All six He i emission lines were used for the $\chi^2$ minimisation and $Y_{\rm wm}$ determination. Horizontal lines show a ratio of 1, and error bars are 1$\sigma$ dispersions.
• Figure 4: (a) Electron densities $N_{\rm e}$(He$^+$) derived from the $\chi^2$ minimisation procedure versus $N_{\rm e}$(S ii) derived from the [S ii] line ratio. Only the five He i$\lambda$3889, $\lambda$4471, $\lambda$5876, $\lambda$6678, and $\lambda$7065 optical emission lines have been used for the $\chi^2$ minimisation. The NIR He i$\lambda$10830 emission line has been excluded. $N_{\rm e}$(He$^+$) has been varied in the range 10 -- 600 cm$^{-3}$. (b) Electron temperature ratios $T_{\rm e}$(He$^+$)/$T_{\rm e}$(O iii) derived from the $\chi^2$ minimisation procedure versus $T_{\rm e}$(O iii) derived from the O iii line ratio. Here, $t_{\rm e}$ = 10$^{-4}$$T_{\rm e}$. Only the five He i$\lambda$3889, $\lambda$4471, $\lambda$5876, $\lambda$6678, and $\lambda$7065 optical emission lines have been used for the $\chi^2$ minimisation. The NIR He i$\lambda$10830 emission line has been excluded. $T_{\rm e}$(He$^+$) has been varied in the range 0.95 -- 1.05 of the $\widetilde{T}_{\rm e}$(He$^+$), where $\widetilde{T}_{\rm e}$(He$^+$) is defined by Eq. \ref{['tHeOIII']}. (c) and (d) Same as (a) and (b), respectively, except that the NIR He i$\lambda$10830 emission line is now included, so that all six He i$\lambda$3889, $\lambda$4471, $\lambda$5876, $\lambda$6678, $\lambda$7065, and $\lambda$10830 emission lines are used for the $\chi^2$ minimisation. 1$\sigma$ errors bars are shown in all panels.
• Figure 5: Same as in Fig. \ref{['fig4']}, but the electron temperature $T_{\rm e}$(He$^+$) is set equal to $\widetilde{T}_{\rm e}$(He$^+$).