Intertwined Formation of $\rm{H_2}$, Dust, and Stars in Cosmological Simulations

Abstract: Context: Molecular hydrogen ($\rm{H_2}$) is crucial in galaxy formation and evolution, serving as the main fuel for star formation (SF). In metal-enriched environments, $\rm{H_2}$ primarily forms on interstellar dust grain surfaces. However, due to the complexities of modelling this process, SF in cosmological simulations often relies on empirical or theoretical frameworks validated only in the Local Universe to estimate the abundance of $\rm{H_2}$. Aims: This study aims to model the connection between star, dust, and $\rm{H_2}$ formation processes in cosmological simulations. Methods: We include $\rm{H_2}$ formation on dust grain surfaces and account for molecule destruction and radiation shielding into the SF and feedback model MUPPI. Results: The model reproduces key properties of observed galaxies for stellar, dust, and $\rm{H_2}$ components. The cosmic density of $\rm{H_2}$ ($\rho_{\rm{H2}}$) peaks around $z=1.5$, then decreases by half towards $z=0$, showing milder evolution than observed. The $\rm{H_2}$ mass function since $z=2$ also shows gentler evolution. Our model successfully recovers the integrated molecular Kennicutt-Schmidt (mKS) law between surface star formation rate ($\Sigma_{\rm SFR}$) and surface $\rm{H_2}$ density ($\Sigma_{\rm H2}$) at $z=0$, already evident at $z=2$ with a higher normalization. We find hints of a broken power law with a steeper slope at higher $\Sigma_{\rm H2}$, aligning with some observational findings. Additionally, the $\rm{H_2}$-to-dust mass ratio in galaxies shows a decreasing trend with gas metallicity and stellar mass. The $\rm{H_2}$-to-dust mass fraction for the global galaxy population is higher at higher redshifts. The analysis of the atomic-to-molecular transition on a particle-by-particle basis suggests that gas metallicity cannot reliably substitute the dust-to-gas ratio in models simulating dust-promoted $\rm{H_2}$.