GRUMPY: a simple framework for realistic forward-modelling of dwarf galaxies

We present a simple regulator-type framework designed specifically for modelling formation of dwarf galaxies. We explore sensitivity of model predictions for the stellar mass--halo mass and stellar mass--metallicity relations to different modelling choices and parameter values. Despite its simplicity, when coupled with realistic mass accretion histories of haloes from simulations and reasonable choices for model parameter values, the framework can reproduce a remarkably broad range of observed properties of dwarf galaxies over seven orders of magnitude in stellar mass. In particular, we show that the model can simultaneously match observational constraints on the stellar mass-halo mass relation, as well as observed relations between stellar mass and gas phase and stellar metallicities, gas mass, size, and star formation rate, as well as general form and diversity of star formation histories (SFHs) of observed dwarf galaxies. The model can thus be used to predict photometric properties of dwarf galaxies hosted by dark matter haloes in $N$-body simulations, such as colors, surface brightnesses, and mass-to-light ratios and to forward model observations of dwarf galaxies. We present examples of such modelling and show that colors and surface brightness distributions of model galaxies are in good agreement with observed distributions for dwarfs in recent observational surveys. We also show that in contrast with the common assumption, the absolute magnitude-halo mass relation is generally predicted to have a non-power law form in the dwarf regime, and that the fraction of haloes that host detectable ultrafaint galaxies is sensitive to reionization redshift (zrei) and is predicted to be consistent with observations for zrei<~9.