On the cosmic evolution of the specific star formation rate
  The apparent correlation between the specific star formation rate (sSFR) and
total stellar mass (M_star) of galaxies is a fundamental relationship
indicating how they formed their stellar populations. To attempt to understand
this relation, we hypothesize that the relation and its evolution is regulated
by the increase in the stellar and gas mass surface density in galaxies with
redshift, which is itself governed by the angular momentum of the accreted gas,
the amount of available gas, and by self-regulation of star formation. With our
model, we can reproduce the specific SFR-M_star relations at z~1-2 by assuming
gas fractions and gas mass surface densities similar to those observed for
z=1-2 galaxies. We further argue that it is the increasing angular momentum
with cosmic time that causes a decrease in the surface density of accreted gas.
The gas mass surface densities in galaxies are controlled by the centrifugal
support (i.e., angular momentum), and the sSFR is predicted to increase as,
sSFR(z)=(1+z)^3/t_H0, as observed (where t_H0 is the Hubble time and no free
parameters are necessary). At z>~2, we argue that star formation is
self-regulated by high pressures generated by the intense star formation
itself. The star formation intensity must be high enough to either balance the
hydrostatic pressure (a rather extreme assumption) or to generate high
turbulent pressure in the molecular medium which maintains galaxies near the
line of instability (i.e. Toomre Q~1). The most important factor is the
increase in stellar and gas mass surface density with redshift, which allows
distant galaxies to maintain high levels of sSFR. Without a strong feedback
from massive stars, such galaxies would likely reach very high sSFR levels,
have high star formation efficiencies, and because strong feedback drives
outflows, ultimately have an excess of stellar baryons (abridged).
