Astrophys. J., 602, 200-230 (2004/February-2)
Systematic effects and a new determination of the primordial abundance of 4He and dY/dZ from observations of blue compact galaxies.
IZOTOV Y.I. and THUAN T.X.
Abstract (from CDS):
We use spectroscopic observations of a sample of 82 H II regions in 76 blue compact galaxies to determine the primordial helium abundance Ypand 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 Te(He II) in the He+ zone and Te(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 Yp=0.2421±0.0021 and dY/dO=5.7±1.8, which corresponds to dY/dZ=3.7±1.2, assuming the oxygen mass fraction to be O=0.66Z. In the framework of the standard big bang nucleosynthesis theory, this Ypcorresponds to Ωbh2=0.012+0.003–0.002, where h is the Hubble constant in units of 100 km/s/Mpc. 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/dO=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.
Galaxies: Abundances - Galaxies: Irregular - Galaxies: ISM - ISM: H II Regions - ISM: Abundances
VizieR on-line data:
<Available at CDS (J/ApJ/602/200): table1.dat refs.dat table3.dat table4.dat table5.dat>
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