Possible flakes of molecular hydrogen in the early Universe

¹ Observatory of Geneva, University of Geneva, 1290 Sauverny, Switzerland e-mail: Daniel.Pfenniger@obs.unige.ch
² Institute of Theoretical Physics, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland e-mail: puy@physik.unizh.ch

Corresponding author: D. Pfenniger, Daniel.Pfenniger@obs.unige.ch

Received: 27 September 2002
Accepted: 3 November 2002

Abstract

The thermochemistry of H₂ and HD in non-collapsed, non-reionized primordial gas up to the end of the dark age is investigated with recent radiation-matter and chemical reaction rates taking into account the efficient coolant HD, and the possibility of a gas-solid phase transition of H₂. In the standard big-bang model we find that these molecules can freeze out and lead to the growth of flakes of solid molecular hydrogen at redshifts in the unperturbed medium and under-dense regions. While this freezing caused by the mere adiabatic cooling of the expanding matter is less likely to occur in collapsed regions due to their higher than radiation background temperature, on the other hand the super-adiabatic expansion in voids strongly favors it. Later reionization (at ) eventually destroys all these H₂ flakes. The possible occurrence of H₂ flakes is important for the degree of coupling between matter and radiation, as well as for the existence of a gas-grain chemistry at the end of the dark age.