The Primordial Helium-4 Abundance from Observations of a Large Sample of Blue Compact Dwarf Galaxies

Abstract

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. N/H linear regressions to O/H = N/H = 0, we obtain Yp = 0.244±0.002 and 0.245±0.001, respectively, higher than previous determinations (Yp = 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 I Zw 18 which, because of its extremely low metallicity plays a key role in the determination of Yp. 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 Yp gives Ω_b h ²₅₀ = 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 ⁷Li in galactic halo stars and with the D/H abundance measured in absorption systems toward quasars by Buries and Tytler (1997).