Abstract We report the presence of a systematic excess in the molecular hydrogen fraction (fH2 = 2 nH2/nH) in studies that use a reduced chemistry network to calculate fH2 of gas with a non-zero metal mass fraction. This is common practice in simulations of galaxy formation in which following the non-equilibrium abundances of additional elements is computationally expensive. We define the H2 excess as the shift in density of the H i-H2 transition in the reduced network compared to the full chemical network (30 elements). The strength of the H2 excess generally increases both with temperature and metallicity, is largely independent of the radiation field strength, and persists across a large range of assumed shielding column densities. For warm gas, with T ≈ 1000 K, the H i-H2 transition is shifted by up to 1 dex to lower densities in primordial chemistry networks already for extremely low metallicities (Z ≥ 10−4 Z⊙). We confirm our earlier findings that missing reactions with oxygen are largely responsible for this H2 excess. A reduced chemical network of hydrogen, helium, and oxygen recovers the molecular hydrogen fractions from a full network and we therefore recommend to include destruction of molecular hydrogen by oxygen in a minimal chemical network for accurate molecular hydrogen abundances.
Sylvia Ploeckinger (Thu,) studied this question.