THE PRIMORDIAL HELIUM ABUNDANCE FROM OBSERVATIONS OF EXTRAGALACTIC H-II REGIONS

The pre-galactic (assumed primordial) helium mass fraction Y(p) is determined to be 0.228 +/- 0.005 (s.e.) or Y(p) < 0.242 with 95 per cent confidence, taking reasonably likely systematic errors into account. This is based on INT and AAT observations of emission lines from H II galaxies combined with selected data from the literature relating to extragalactic H II regions in general, discussed in a consistent analysis taking into account the known corrections and likely sources of error. Maximum-likelihood regressions of helium against both oxygen and nitrogen are found to be quite accurately linear up to 10(6) O/H = 240 and 10(7) N/H = 66, respectively, with a dY\dZ slope close to 5 (or about 4 if internal dust is significant), provided that H II galaxies known to have broad WR emission features (N III 4634-42 and broad He II 4686) are excluded; the latter often show upward deviations from the helium abundance found for other objects with the same oxygen and sometimes with the same nitrogen abundance. This supports a suggestion made earlier that there may be additional local sources of both helium and nitrogen in the form of winds from Wolf-Rayet stars (or their red supergiant progenitors). Detailed observations of NGC 5253 indicate, however, that local enhancements of helium are not always co-extensive in space with those of nitrogen.