ИСТИНА

On the example of concrete spiral galaxies it will be demonstrated the existence of well-defined connection between the efficiency of star formation (SFE) and stellar volume density ρ∗ in the disc midplane, calculated for the equilibrium axisymmetric disc models. However this correlation disappears at the disc peripheries, where rho<10^(−24) g/cm^3. The relationship between SFE and the azimuthally averaged volume density of HI is more diffuse, evidently because of the irregular gas distribution. A possible role of various mechanisms which may be responsible for local growth of gas density and star formation in the low-dense gas layer will be shortly discussed, such as density waves, induced by non-axisymmetric dark halo, the existence of CO-silent molecular gas clouds, or the effects of environment.