Systematic effects and self-consistency in the primordial helium abundance determination

Observations of light element abundances (⁴He, D, ⁷Li) provide a powerful and independent test of the Standard Model of Big Bang Nucleosynthesis. The ⁴He abundance is measured most accurately among the light elements. This value is determined from observations of spectral lines from small-scale extragalactic sources (HII regions) in the optical and near infrared ranges. The intensity of the forbidden lines like [OIII] and [SII] allows to obtain the physical characteristics of the source, such as temperature and metallicity. Intensities of the ⁴He lines are used to determine the abundance of this element in the object. However, there are various systematic effects that should be taken in account for precise definition of this physical quantity. These include (1) interstellar reddening, (2) emissivities of ⁴He in different wavelengths, (3) underlying absorption for H and He lines, (4) temperature and (5) ionization structure HII regions, (6) collisional and fluorescent excitation of H and He lines. Incorrect accounting of these effects can lead to overestimation or underestimation of the primordial helium abundance Y _p by several percent.

At the moment, there are two independent groups of researchers dealing with this problem. The results for the calculation using the same data sample for these groups are as follows: Y_p = 0.2551 ± 0.0022 (Izotov, Thuan, Guseva, 2014) and Y _p = 0.2449 ± 0.0040 (Aver, Olive, Skillman, 2015). The last result is consistent with the result Y _p = 0.2467 ± 0.0006 based on the study of the CMB anisotropy on (P.A.R. Ade et al., 2015). However, the values of Y _p obtained by these groups do not coincide even within the margin of error, and at the moment there is no clear answer to the question of which of these two groups got correct result. In the paper, differences in the methods of processing data and obtaining the quantity of the primordial helium abundance are investigated, in particular, how the groups take into account the named systemic effects. An error analysis and comparative tests are performed. (paper)