Blue compact galaxies and the primordial 4Helium abundance

Abstract

Blue compact galaxies (BCG) are ideal objects in which to derive the primordial 4He abundance because they are chemically young and have not had a significant stellar He contribution. We discuss a self-consistent method which makes use of all the brightest He I emission lines in the optical range and solves consistently for the electron density of the He II zone. We pay particular attention to electron collision and radiative transfer as well as underlying stellar absorption effects which may make the He I emission lines deviate from their recombination values. Using a large homogeneous sample of 45 low-metallicity H II regions in BCGs, and extrapolating the Y-O/H and Y-N/H linear regressions to O/H = N/H = 0, we obtain Yp = 0.2443+/-0.0015, in excellent agreement with the weighted mean value Yp = 0.2452+/-0.0015 obtained from the detailed analysis of the two most metal-deficient BCGs known, I Zw 18 and SBS 0335-052. The derived slope dY/dZ = 2.4+/-1.0 is in agreement with the value derived for the Milky Way and with simple chemical evolution models with homogeneous outflows. Adopting Yp = 0.2452+/-0.0015 leads to a baryon-to-photon ratio of (4.7+/-1.0)x10^{-10} and to a baryon mass fraction in the Universe Omega_b h^2_{50} = 0.068+/-0.015, consistent with the value derived from the primordial D abundance of Burles & Tytler (1998).