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We calculated the radial profiles of the azimuthally averaged midplane gasvolume density $\rho_g$ for 11 high surface brightness (HSB) spiral galaxies, 7low surface brightness (LSB) galaxies and 3 S0 galaxies assuming their gaseouslayers to be in the equilibrium state in the plane of marginally stable stellardiscs. We compared the surface star formation rate ($\Sigma_{SFR}$) and starformation efficiency ($SFE=\Sigma_{SFR}/\Sigma_{gas}$) with $\rho_g$ andstellar surface density $\Sigma_s$ assuming the latter to be proportional todisc surface brightness. Both HSB and LSB galaxies follow a single sequence$\Sigma_{SFR}-\rho_{g}$ and $SFE-\Sigma_s$ or $SFE-\rho_s$. It means that theconditions of star formation are similar in the outer discs of normal spiralgalaxies and in the inner regions of LSB galaxies if their stellar discs havesimilar densities. The relationship between $SFE$ and $\rho_s$ is close to thelaw $SFE$ $\sim \rho_s^{1/2}$ expected in the theoretical model ofself-regulated star formation proposed by Ostriker et al.\cite{OstrikerAll10}.The alternative explanation is to propose that $SFE$ is proportional to afrequency of vertical oscillation of gas clouds around the disc midplane. Inthe most rarified regions of LSB galaxies the efficiency of star formation isnearly independent on gas and stellar disc densities being higher in the meanthan it is expected from the extrapolation of the power law fit for HSB samplegalaxies. Evidently in these regions with extremely low $\rho_{g}$ SFE dependson local density fluctuations rather than on the azimuthally averaged discparameters.

Gas density and star formation in the rarified regions of discs of normal and LSB galaxies