Abstract
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. 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, 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, tau_eq, which is determined by the
product of star-formation efficiency 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 are primarily governed by tau_eq. Most strikingly, the predicted sSFR
evolution is controlled solely by tau_eq (apart from the cosmic time). 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 four. 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*sMIR_DM. The predicted sSFR from the gas regulator model is in good
agreement with 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 that are very unlikely to live around the equilibrium state at any
epoch and this has been largely ignored in many similar studies.
Users
Please
log in to take part in the discussion (add own reviews or comments).