Reassessing the primordial helium abundance - New observations of NGC 4861 and CG 1116 + 51

Abstract

New spectrophotometric observations of the metal-poor H II regions in the dwarf irregular galaxies NGC 4861 and CG 1116 + 51 are reported along with an analysis of their emission-line intensities utilizing nebular photoionization models. In contrast with previous results for these two galaxies, it is found that their helium mass fractions Y are consistent with results for other metal-poor dwarf galaxies and with standard big bang cosmological models. Previous evidence for large variations in Y among galaxies with low abundances of heavy elements are not confirmed. Fluctuations in the primordial value of Y were probably smaller than the current measurement and analysis uncertainty of about 10 percent. Further progress on this problem will require better SNR measurements on the weaker He lines. Better constraints on the correction for neutral He are crucial.