The Impact of cold gas accretion above a mass floor on galaxy scaling
  relations
  Using the cosmological baryonic accretion rate and normal star formation
efficiencies, we present a very simple model for star-forming galaxies (SFGs)
that accounts for the mass and redshift dependencies of the SFR-Mass and
Tully-Fisher relations from z=2 to the present. The time evolution follows from
the fact that each modelled galaxy approaches a steady state where the SFR
follows the (net) cold gas accretion rate. The key feature of the model is a
halo mass floor M_{min}~10^{11} below which accretion is quenched in order to
simultaneously account for the observed slopes of the SFR-Mass and
Tully-Fischer relations. The same successes cannot be achieved via a
star-formation threshold (or delay) nor by varying the SF efficiency or the
feedback efficiency. Combined with the mass ceiling for cold accretion due to
virial shock heating, the mass floor M_{min} explains galaxy "downsizing",
where more massive galaxies formed earlier and over a shorter period of time.
It turns out that the model also accounts for the observed galactic baryon and
gas fractions as a function of mass and time, and the cosmic SFR density from
z~6 to z=0, which are all resulting from the mass floor M_{min}. The model
helps to understand that it is the cosmological decline of accretion rate that
drives the decrease of cosmic SFR density between z~2 and z=0 and the rise of
the cosmic SFR density allows us to put a constraint on our main parameter
M_{min}~10^{11} solar masses. Among the physical mechanisms that could be
responsible for the mass floor, we view that photo-ionization feedback (from
first in-situ hot stars) lowering the cooling efficiency is likely to play a
large role.
