From haloes to Galaxies - I. The dynamics of the gas regulator model and the implied cosmic sSFR history

We explore the basic parameters that drive the evolution of the fundamental properties of star-forming galaxies within the 'gas regulator model', or bathtub-model. From the five basic equations of the typical gas regulator model, we derive the general analytic form of the evolution of the key galaxy properties, i.e. gas mass, star formation rate (SFR), stellar mass, specific SFR (sSFR), gas fraction, gas phase metallicity and stellar metallicity, without assuming that galaxies live in the equilibrium state. We find that the timescale required to reach equilibrium, t eq, which is determined by the product of star formation efficiency e and mass-loading factor., is the central parameter in the gas regulator model that is essentially in control of the evolution of all key galaxy properties. The scatters in most of the key scaling relations, such as the stellar mass-SFR relation and stellar mass-metallicity relation, are primarily governed by t eq. Most strikingly, the predicted sSFR evolution is controlled solely by tau(eq) (apart from the cosmic time), independent of the gas inflow rate and of the individual values of epsilon and lambda. Although the precise evolution of the sSFR depends on tau(eq), the sSFR history is largely insensitive to different values of tau(eq). The difference between the minimum and maximum sSFR at any epoch is less than a factor of 4 for any given values of tau(eq). The shape of the predicted sSFR history simply mimics that of the specific mass increase rate of the dark matter halos (sMIR(DM)) with the typical value of the sSFR around 2 x sMIR(DM). We show that the predicted sSFR from the gas regulator model is in good agreement with the predictions from typical semi-analytic models (SAMs), but both are fundamentally different from the observed sSFR history. This clearly implies that some key process is missing in both typical SAMs and gas regulator model, and we hint at some possible culprit. We emphasize the critical role of tau(eq) in controlling the evolution of the galaxy population, especially for gas rich low-mass galaxies and dwarf galaxies that are very unlikely to live around the equilibrium state at any epoch and this has been largely ignored in many similar studies.