The mass-dependent star formation histories of disk galaxies: infall
  model versus observations
  We introduce a simple model to explore the star formation histories of disk
galaxies. We assume that the disk origins and grows by continuous gas infall.
The gas infall rate is parametrized by the Gaussian formula with one free
parameter: infall-peak time $t_p$. The Kennicutt star formation law is adopted
to describe how much cold gas turns into stars. The gas outflow process is also
considered in our model. We find that, at given galactic stellar mass $M_*$,
model adopting late infall-peak time $t_p$ results in blue colors, low
metallicity, high specific star formation rate and high gas fraction, while gas
outflow rate mainly influences the gas-phase metallicity and star formation
efficiency mainly influences the gas fraction. Motivated by the local observed
scaling relations, we construct a mass-dependent model by assuming low mass
galaxy has later infall-peak time $t_p$ and larger gas outflow rate than
massive systems. It is shown that this model can be in agreement with not only
the local observations, but also the observed correlations between specific
star formation rate and galactic stellar mass $SFR/M_* \sim M_*$ at
intermediate redshift $z<1$. Comparison between the Gaussian-infall model and
exponential-infall model is also presented. It shows that the
exponential-infall model predicts higher star formation rate at early stage and
lower star formation rate later than that of Gaussian-infall. Our results
suggest that the Gaussian infall rate may be more reasonable to describe the
gas cooling process than the exponential infall rate, especially for low-mass
systems.
