The primordial helium abundance from observations of extragalactic H II regions

The pre-galactic (assumed primordial) helium mass fraction YP is determined to be |0. 2280. 005\, (s. e. ) \, or\, YP0. 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.