Constraints on the primordial helium abundance from the optical emission spectra of dwarf galaxies

At present, the best estimate of the primordial helium abundance (Ysub(p)) comes from helium abundances that have been derived from the emission spectra of dwarf galaxies. The mean Ysub(p) deduced from the abundances in the 26 best observed dwarf galaxies is ca. 0.24. Individual galaxies may show a somewhat smaller Ysub(p) (ca. 0.22) but the errors are uncertain and confirming observations are needed. It is important to search for more dwarf galaxies like IZw18 (with very low metal abundances) for further determinations of helium abundance. (author)