The Primordial Abundance of4He Revisited

Abstract

We use a sample of 45 low-metallicity H II regions to determine the primordial helium abundance Y p with a precision of better than 1%. The data includes new spectrophotometric observations of 15 blue compact galaxies (BCGs) with oxygen abundance 12 ) log (O/H) between 7.83 and 8.35 (Z _ /13 Z most of which were selected from the First Byurakan and the University of Michigan objective Z _ /4), prism surveys. We have included many low-metallicity BCGs in our sample, including the two most metal-de-cient galaxies known, I Zw 18 and SBS 0335(052 We have carefully investi- (Z _ /55) (Z _ /43). gated the physical e†ects that may make the He I line intensities deviate from their recombination values, such as collisional and Nuorescent enhancements, underlying He I stellar absorption, and absorp- tion by Galactic interstellar Na I. By extrapolating the Y versus O/H and Y versus N/H linear regres- sions to O/H N/H 0, we obtain and 0.245 ^ 0.001, respectively, in agreement Y p 0.244 ^ 0.002 with the study of Izotov, Thuan, & Lipovetsky, but higher than previous determinations (Y p 0.230E0.234). Part of the di†erence comes from the fact that previous investigators have used the north- west component of I Zw 18 in the determination of This component is subject to strong underlying Y p . He I stellar absorption that reduces the He I line intensities by 5%E25%. The derived Y is 0.233 ^ 0.008 from the He I j6678 line. Instead, by using the southeast component of I Zw 18, which is much less subject to underlying He I stellar absorption, we obtain Y 0.242 ^ 0.009. The mean Y of the two most metal-de-cient BCGs, I Zw 18 and SBS 0335(052, is in excellent agreement with the Y1 0.245 ^ 0.004, derived from the linear regressions. We derive a slope dY /dZ 2.3 ^ 1.0, considerably smaller than Y p those derived before. With this smaller slope and taking into account the errors, chemical evolution models with an outNow of well-mixed material can be built for star-forming dwarf galaxies that satisfy all the observational constraints. Our gives consistent with the lower limit set Y p ) b h 50 0.058 ^ 0.007, 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 at the 1 p level with the D/H abundance measured in absorption systems toward quasars by Tytler & Burles. Subject headings: galaxies: abundances E galaxies: irregular E galaxies: ISM E H II regions E ISM: abundances