From atoms to stars: Modelling H 2 formation and its impact on galactic evolution

Abstract

We present a sub-grid model for star formation in galaxy simulations, incorporating molecular hydrogen (H2) production via dust grain condensation and its destruction through star formation and photodissociation. Implemented within the magnetohydrodynamical code AREPO, our model tracks the non-equilibrium mass fractions of molecular, ionised, and atomic hydrogen, as well as a stellar component, by solving a system of differential equations governing mass exchange between these phases. Star formation is treated with a variable rate dependent on the local H2 abundance, which itself varies in a complex way with key quantities such as gas density and metallicity. Testing the model in a cosmological simulation of a Milky Way-mass galaxy, we obtain a well-defined spiral structure at z=0, including a gas disc twice the size of the stellar one, alongside a realistic star formation history. Our results show a broad range of star formation efficiencies per free-fall time, from as low as 0.001% at high redshift to values between 0.1% and 10% for ages ≳ 3−4 Gyr. These findings align well with observational estimates and simulations of a turbulent interstellar medium. Notably, our model reproduces a star formation rate versus molecular hydrogen surface densities relation akin to the molecular Kennicutt-Schmidt law. Furthermore, we find that the star formation efficiency varies with density and metallicity, providing an alternative to fixed-efficiency assumptions and enabling comparisons with more detailed star formation models. Comparing different star formation prescriptions, we find that in models that link star formation to H2, star formation onset is ∼ 500 Myr later than those relying solely on total or cold gas density.