The primordial helium-4 abundance from observations of a large sample of blue compact dwarf galaxies

We use a sample of 45 low-metallicity H II regions in blue compact dwarf (BCD) galaxies to determine the primordial helium abundance Yp With a precision better than 5%. We have carefully investigated the physical effects which may make the He I line intensities deviate from their recombination values such as collisional and fluorescent enhancements, underlying He I stellar absorption and absorption by Galactic interstellar Na I. By extrapolating the Y vs. O/H and Y vs. NIH linear regressions to O/H = NIH = 0, we obtain Y-P = 0.244+/-0.002 and 0.245+/-0.001, respectively, higher than previous determinations (Y-P = 0.230 - 0.234). Part of the difference comes from the fact that previous investigators have not taken into account underlying He I stellar absorption, especially in the NW component of the BCD IZw 18 which, because of its extremely low metallicity plays a key role in the determination of Y-P. We derive a slope dY/dZ = 2.3+/-1.0, considerably smaller than those derived before. With this smaller slope and taking into account the errors, chemical evolution models with an outflow of well-mixed material can be built for star-forming dwarf galaxies which satisfy all the observational constraints. Our Y-P gives Ohm(b)h(50)(2) = 0.058+/-0.007, consistent with the lower limit set by dynamical measurements and X-ray observations of clusters of galaxies. It is also consistent, within the framework of standard big bang nucleosynthesis theory, with measurements of primordial Li-7 in galactic halo stars and with the D/H abundance measured in absorption systems toward quasars by Buries and Tytler(1997).