Towards the complete census of molecular hydrogen in a simulated disc galaxy

We present a multiscale analysis of molecular hydrogen in a Milky Way-like simulated galaxy. Our census covers the gas content of the entire disc, to radial profiles and the Kennicutt-Schmidt relation, to a study of its molecular clouds, and finally down to a cell-by-cell analysis of the gas phases. A significant fraction of the H₂ gas is in low-density regions mixed with atomic hydrogen and would therefore be difficult to observe. We use the molecular addition to RAMSES-RT, an adaptive mesh refinement grid code with the hydrodynamics coupled to moment-based radiative transfer. Three resolutions of the same galaxy detail the effects it has on H₂ formation, with grid cells sized 97, 24, and 6.1 pc. Only the highest resolution yields gas densities high enough to host significant H₂ fractions, and resolution is therefore key to simulating H₂. Our H₂ content is not completely converged but we find general agreement with available observations. Apart our pieces of galactic analysis are disparate, but assembled, they provide a cohesive portrait of H₂ in the interstellar medium. H₂ chemistry on the atomic scale is sufficient to generate its dynamics throughout an entire galaxy.