HD/H₂ molecular clouds in the early Universe: The problem of primordial deuterium

Abstract

We have detected new HD absorption systems at high redshifts, z _abs = 2.626 and z _abs = 1.777, identified in the spectra of the quasars J0812+3208 and Q1331+170, respectively. Each of these systems consists of two subsystems. The HD column densities have been determined: log N {_HD/^A} = 15.70 ± 0.07 for z _A = 2.626443(2) and log N {_HD/^B} = 12.98 ± 0.22 for z _B = 2.626276(2) in the spectrum of J0812+3208 and log N {_HD/^C} = 14.83 ± 0.15 for z _C = 1.77637(2) and log N {_HD/^D} = 14.61 ± 0.20 for z _D = 1.77670(3) in the spectrum of Q1331+170. The measured HD/H₂ ratio for three of these subsystems has been found to be considerably higher than its values typical of clouds in our Galaxy.We discuss the problem of determining the primordial deuterium abundance, which is most sensitive to the baryon density of the Universe Ω_b. Using a well-known model for the chemistry of a molecular cloud, we have estimated the isotopic ratio D/H=HD/2H₂ = (2.97 ± 0.55) × 10^-5 and the corresponding baryon density Ω_b h ² = 0.0205{_-0.0020/^+0.0025}. This value is in good agreement with Ω_b h ² = 0.0226{_-0.0006/^0.0006} obtained by analyzing the cosmic microwave background radiation anisotropy. However, in high-redshift clouds, under conditions of low metallicity and low dust content, hydrogen may be incompletely molecularized even in the case of self-shielding. In this situation, the HD/2H₂ ratio may not correspond to the actual D/H isotopic ratio. We have estimated the cloud molecularization dynamics and the influence of cosmological evolutionary effects on it.