H$_2$-based star formation laws in hierarchical models of galaxy formation

TL;DR

This paper updates a galaxy evolution model to include detailed gas partitioning and star formation laws, showing that while global galaxy properties are robust, the molecular-to-atomic hydrogen ratio varies significantly with different models, especially at high redshift.

Contribution

The study introduces a self-consistent treatment of atomic and molecular gas partitioning in a galaxy formation model, improving its predictive power and exploring the impact of various star formation laws.

Findings

Model reproduces observed galaxy stellar mass functions.

Gas metallicity evolution matches observations across redshifts.

Molecular-to-atomic hydrogen ratio varies with star formation law, especially at high redshift.

Abstract

We update our recently published model for GAlaxy Evolution and Assembly (GAEA), to include a self-consistent treatment of the partition of cold gas in atomic and molecular hydrogen. Our model provides significant improvements with respect to previous ones used for similar studies. In particular, GAEA (i) includes a sophisticated chemical enrichment scheme accounting for non-instantaneous recycling of gas, metals, and energy; (ii) reproduces the measured evolution of the galaxy stellar mass function; (iii) reasonably reproduces the observed correlation between galaxy stellar mass and gas metallicity at different redshifts. These are important prerequisites for models considering a metallicity dependent efficiency of molecular gas formation. We also update our model for disk sizes and show that model predictions are in nice agreement with observational estimates for the gas, stellar and star forming disks at different cosmic epochs. We analyse the influence of different star formation laws including empirical relations based on the hydro-static pressure of the disk, analytic models, and prescriptions derived from detailed hydro-dynamical simulations. We find that modifying the star formation law does not affect significantly the global properties of model galaxies, neither their distributions. The only quantity showing significant deviations in different models is the cosmic molecular-to-atomic hydrogen ratio, particularly at high redshift. Unfortunately, however, this quantity also depends strongly on the modelling adopted for additional physical processes. Useful constraints on the physical processes regulating star formation can be obtained focusing on low mass galaxies and/or at higher redshift. In this case, self-regulation has not yet washed out differences imprinted at early time.