The excess of molecular hydrogen in chemical networks without oxygen

TL;DR

This study identifies a systematic overestimation of molecular hydrogen in simplified chemical networks used in galaxy formation simulations, caused by missing oxygen reactions, and proposes including oxygen destruction processes for accuracy.

Contribution

The paper demonstrates that missing oxygen reactions cause an excess of molecular hydrogen in reduced networks and recommends including oxygen destruction for accurate modeling.

Findings

H2 excess increases with temperature and metallicity

H2 excess persists across various shielding column densities

Including oxygen destruction recovers accurate H2 fractions

Abstract

We report the presence of a systematic excess in the molecular hydrogen fraction ($f_{\mathrm{H2}} = 2 \, n_{\mathrm{H2}}/n_{\mathrm{H}}$) in studies that use a reduced chemistry network to calculate $f_{\mathrm{H2}}$ 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 $\mathrm{H}_2$ excess as the shift in density of the \ion{H}{I}-$\mathrm{H}_2$ transition in the reduced network compared to the full chemical network (30 elements). The strength of the $\mathrm{H}_2$ 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 $T\approx1000~\mathrm{K}$, the HI-$\mathrm{H}_2$ transition is shifted by up to 1 dex to lower densities in primordial chemistry networks already for extremely low metallicities ($Z\geq 10^{-4}\,\mathrm{Z}_{\odot}$). We confirm our earlier findings that missing reactions with oxygen are largely responsible for this $\mathrm{H}_2$ 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.