  The Schmidt-Kennicutt relation links the surface densities of gas to the star
formation rate in galaxies. The physical origin of this relation, and in
particular its break, i.e. the transition between an inefficient regime at low
gas surface densities and a main regime at higher densities, remains debated.
Here, we study the physical origin of the star formation relations and breaks
in several low-redshift galaxies, from dwarf irregulars to massive spirals. We
use numerical simulations representative of the Milky Way, the Large and the
Small Magellanic Clouds with parsec up to subparsec resolution, and which
reproduce the observed star formation relations and the relative variations of
the star formation thresholds. We analyze the role of interstellar turbulence,
gas cooling, and geometry in drawing these relations, at 100 pc scale. We
suggest in particular that the existence of a break in the Schmidt- Kennicutt
relation could be linked to the transition from subsonic to supersonic
turbulence and is independent of self-shielding effects. This transition being
connected to the gas thermal properties and thus to the metallicity, the break
is shifted toward high surface densities in metal-poor galaxies, as observed in
dwarf galaxies. Our results suggest that together with the collapse of clouds
under self-gravity, turbulence (injected at galactic scale) can induce the
compression of gas and regulate star formation.
