  CO observations of the interacting galaxies IC 2163 and NGC 2207 are combined
with HI, Halpha and 24 microns to study the star formation rate (SFR) surface
density as a function of the gas surface density. More than half of the high
SFR regions are HI dominated. When compared to other galaxies, these
HI-dominated regions have excess SFRs relative to their molecular gas surface
densities but normal SFRs relative to their total gas surface densities. The
HI-dominated regions are mostly located in the outer part of NGC 2207, where
the HI velocity dispersion is high, 40 - 50 km/s. We suggest that the
star-forming clouds in these regions have envelopes at lower densities than
normal, making them predominantly atomic, and cores at higher densities than
normal because of the high turbulent Mach numbers. This is consistent with
theoretical predictions of a flattening in the density probability distribution
function for compressive, high Mach number turbulence.
