A new determination of the primordial He abundance using the He i λ10830 Å emission line: cosmological implications

• Published in: Monthly notices of the Royal Astronomical Society 2014-11, Vol.445 (1), p.778-793
• Main Authors: Izotov, Y. I., Thuan, T. X., Guseva, N. G.
• Format: Article
• Language: English
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stu1771

We present near-infrared (NIR) spectroscopic observations of the high-intensity He i λ10830 Å emission line in 45 low-metallicity H ii regions. We combined these NIR data with spectroscopic data in the optical range to derive the primordial He abundance. The use of the He i λ10830 Å line, the intensity of which is very sensitive to the density of the H ii 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 H ii regions with Hβ equivalent width EW(Hβ) ≥ 150 Å, excitation parameter O2 +/O ≥ 0.8, and with helium mass fraction Y derived with an accuracy better than 3 per cent. With this final sample we derived a primordial 4He mass fraction Y p = 0.2551 ± 0.0022. The derived value of Y p is higher than the one predicted by the standard big bang nucleosynthesis model. Using our derived Y p together with D/H = (2.53 ± 0.04) × 10−5, and the χ2 technique, we found that the best agreement between these light element abundances is achieved in a cosmological model with a baryon mass density Ωb h 2 = 0.0240 ± 0.0017 (68 per cent confidence level, CL), ± 0.0028 (95.4 per cent CL), ± 0.0034 (99 per cent CL) and an effective number of neutrino species N eff = 3.58 ± 0.25 (68 per cent CL), ± 0.40 (95.4 per cent CL), ± 0.50 (99 per cent CL). A non-standard value of N eff is preferred at the 99 per cent CL, implying the possible existence of additional types of neutrino species.