  The radial profiles of gas, stars, and far ultraviolet radiation in 20 dwarf
Irregular galaxies are converted to stability parameters and scale heights for
a test of the importance of two-dimensional (2D) instabilities in promoting
star formation. A detailed model of this instability involving gaseous and
stellar fluids with self-consistent thicknesses and energy dissipation on a
perturbation crossing time give the unstable growth rates. We find that all
locations are effectively stable to 2D perturbations, mostly because the disks
are thick. We then consider the average volume densities in the midplanes,
evaluated from the observed HI surface densities and calculated scale heights.
The radial profiles of the star formation rates are equal to about 1% of the HI
surface densities divided by the free fall times at the average midplane
densities. This 1% resembles the efficiency per unit free fall time commonly
found in other cases. There is a further variation of this efficiency with
radius in all of our galaxies, following the exponential disk with a scale
length equal to about twice the stellar mass scale length. This additional
variation is modeled by the molecular fraction in a diffuse medium using
radiative transfer solutions for galaxies with the observed dimensions and
properties of our sample. We conclude that star formation is activated by a
combination of three-dimensional gaseous gravitational processes and molecule
formation. Implications for outer disk structure and formation are discussed.
