The Cosmic Baryon Fraction and the Extragalactic Ionizing Background

Preprint
Publisher Website
Google Scholar
  • 27 April 2001
Abstract
We reassess constraints on the cosmological baryon density from observations of the mean decrement and power spectrum of the Lyman-alpha forest, taking into account uncertainties in all free parameters in the gravitational instability model. The uncertainty is dominated by that of the photoionizing background, but incomplete knowledge of the thermal state of the intergalactic medium also contributes significantly to the error-budget. Current estimates of the baryon fraction prefer values that are somewhat higher than the big bang nucleosynthesis (BBN) value of Omega_b h^2 = 0.0189 \pm 0.0019. However, even assuming the highest estimate of the ionizing background, as indicated by recent measurements of a large escape fraction from Lyman-break galaxies by Steidel, Pettini & Adelberger, we find Omega_b h^2 = 0.039 \pm 0.01, i.e. the forest constraint is inconsistent with the BBN value only at the 97 % (or about 2.2-sigma) level. We find a similar bound using measurements of the ionizing background from the proximity effect. We also develop a formalism which treats lower bounds on the baryon density in a statistical manner, which is appropriate if only a lower bound on the ionizing background is known. Finally, we discuss the implications of the escape fraction measurement for the age, structure and stellar content of Lyman-break galaxies.

This publication has 0 references indexed in Scilit: