Systematic Effects and a New Determination of the Primordial Abundance of4He and dY/dZ from Observations of Blue Compact Galaxies

Abstract

We use spectroscopic observations of a sample of 82 H II regions in 76 blue compact galaxies to determine the primordial helium abundance Y p and the slope dY / dZ from the Y -O/H linear regression. To improve the accuracy of the dY / dZ measurement, we have included new spectrophotometric observations of 33 H II regions that span a large metallicity range, with oxygen abundance 12 + log(O/H) varying between 7.43 and 8.30 ( Z ☉ /30 ≤ Z ≤ Z ☉ /4). Most of the new galaxies were selected from the First Byurakan, the Hamburg/SAO, and the University of Michigan objective prism surveys. For a subsample of seven H II regions, we derive the He mass fraction taking into account known systematic effects, including collisional and fluorescent enhancements of He I emission lines, collisional excitation of hydrogen emission, underlying stellar He I absorption, and the difference between the temperatures T e (He II) in the He + zone and T e (O III) derived from the collisionally excited [O III] lines. We find that the net result of all the systematic effects combined is small, changing the He mass fraction by less than 0.6%. By extrapolating the Y versus O/H linear regression to O/H = 0 for seven H II regions of this subsample, we obtain Y p = 0.2421 ± 0.0021 and dY / d O = 5.7 ± 1.8, which corresponds to dY / dZ = 3.7 ± 1.2, assuming the oxygen mass fraction to be O = 0.66 Z . In the framework of the standard big bang nucleosynthesis theory, this Y p corresponds to Ω b h 2 = 0.012 , where h is the Hubble constant in units of 100 km s -1 Mpc -1 . This is smaller at the 2 σ level than the value obtained from recent deuterium abundance and microwave background radiation measurements. The linear regression slope dY / d O = 4.3 ± 0.7 (corresponding to dY / dZ = 2.8 ± 0.5) for the whole sample of 82 H II regions is similar to that derived for the subsample of seven H II regions, although it has a considerably smaller uncertainty.