H2/HD molecular clouds in the early universe. An independent means of estimating the baryon density of the universe

Areview ofmolecular hydrogen H2 absorption systems identified in quasar spectra is presented. The analysis of such systems allows the determination of the chemical composition of the interstellar medium and the physical conditions existing in the early Universe, about 10–12 billioin years ago. To date, 27 molecular hydrogen systems have been found, nine of which show HD lines. An independent method for estimating the baryon density of the Universe is described, and is based on the analysis of the relative abundances of H2 and HD molecules. Among known H2/HD systems, only the two systems detected in Q1232+082 and Q0812+320 quasar spectra satisfy the condition of self-shielding of the absorbing cloud log \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$N_{H_2 HD} \underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\thicksim}$}}{ > } 15$\end{document}. Under these conditions the local molecular fraction can reach unity, making it possible to estimate the relative deuterium abundance D/H using the ratio of the HD and H2 column densities N(HD)/2N(H2). The analysis of the column densities for these two systems yields D/H = HD/2H2 = (3.26 ± 0.29) × 10−5. Comparison of this result with the prediction of BBN theory for D/H enables the determination of the baryon density of the Universe: Ωbh2 = (0.0194 ± 0.0011). This is somewhat lower than the values Ωbh2 = (0.0224 ± 0.0012) and (0.0221 ± 0.0003) obtained using other independent methods: (i) analysis of the relative D and H abundances in Lyman Limit Systems at high redshifts, and (ii) analysis of the anisotropy of the cosmic microwave background. Nevertheless, all three values agree within their 2σ errors.