On the Star Formation Law for Spiral and Irregular Galaxies
  A dynamical model for star formation on a galactic scale is proposed in which
the interstellar medium is constantly condensing to star-forming clouds on the
dynamical time of the average midplane density, and the clouds are constantly
being disrupted on the dynamical time scale appropriate for their higher
density. In this model, the areal star formation rate scales with the 1.5 power
of the total gas column density throughout the main regions of spiral galaxies,
and with a steeper power, 2, in the far outer regions and in dwarf irregular
galaxies because of the flaring disks. At the same time, there is a molecular
star formation law that is linear in the main and outer parts of disks and in
dIrrs because the duration of individual structures in the molecular phase is
also the dynamical time scale, canceling the additional 0.5 power of surface
density. The total gas consumption time scales directly with the midplane
dynamical time, quenching star formation in the inner regions if there is no
accretion, and sustaining star formation for ~100 Gyr or more in the outer
regions with no qualitative change in gas stability or molecular cloud
properties. The ULIRG track follows from high densities in galaxy collisions.
