The relation between mid-plane pressure and molecular hydrogen in galaxies: Environmental dependence

TL;DR

This study investigates how the empirical correlation between mid-plane pressure and molecular hydrogen abundance in galaxies depends on environmental factors, using hydrodynamical simulations to test its universality and variations.

Contribution

The paper demonstrates that the pressure -- H2 relation naturally emerges in simulations and varies systematically with ISM properties, highlighting environmental dependence.

Findings

The pressure -- H2 relation is consistent with observations at solar metallicity.

The relation's parameters vary with dust-to-gas ratio and radiation field strength.

Lower dust-to-gas ratios lead to decreased normalization of the relation.

Abstract

Molecular hydrogen (H2) is the primary component of the reservoirs of cold, dense gas that fuel star formation in our galaxy. While the H2 abundance is ultimately regulated by physical processes operating on small scales in the interstellar medium (ISM), observations have revealed a tight correlation between the ratio of molecular to atomic hydrogen in nearby spiral galaxies and the pressure in the mid-plane of their disks. This empirical relation has been used to predict H2 abundances in galaxies with potentially very different ISM conditions, such as metal-deficient galaxies at high redshifts. Here, we test the validity of this approach by studying the dependence of the pressure -- H2 relation on environmental parameters of the ISM. To this end, we follow the formation and destruction of H2 explicitly in a suite of hydrodynamical simulations of galaxies with different ISM parameters. We find that a pressure -- H2 relation arises naturally in our simulations for a variety of dust-to-gas ratios or strengths of the interstellar radiation field in the ISM. Fixing the dust-to-gas ratio and the UV radiation field to values measured in the solar neighborhood results in fair agreement with the relation observed in nearby galaxies with roughly solar metallicity. However, the parameters (slope and normalization) of the pressure -- H2 relation vary in a systematical way with ISM properties. A particularly strong trend is the decrease of the normalization of the relation with a lowering of the dust-to-gas ratio of the ISM. We show that this trend and other properties of the pressure -- H2 relation are natural consequences of the transition from atomic to molecular hydrogen with gas surface density.