Star formation efficiency in low-density regions in galactic disks

The radial dependences of the star formation efficiency??_SFE = ??_SFR/??_gas (per unit disk surface area) in normal surface brightness spiral galaxies and low surface brightness (LSB) galaxies are compared with the radial variations of the gas and stellar disk surface and volume densities. The volume density of the components in the disk midplane is found through a self-consistent solution of the disk equilibrium equations by taking into account the dark halo. The disk thickness variation with radius R is calculated within the model of a galaxy with a marginally stable disk by taking into account the increase of the stability parameter Q _T,c along the radius. We show that the star formation efficiency depends weakly (for LSB galaxies, does not depend at all) on the gas density but correlates well with the disk surface and volume density, with the normal and LSB galaxies forming a single sequence. The dependence vanishes only at extremely low disk densities (?? _disk ? (1?3) M _?? pc^?2, ?? _stars ?? (1?3) × 10^?24 g cm^?3), where star formation probably ceases to be related to disk properties. Estimations of the gas volume density allow us to check the expected form of the ??_SFR-?? _disk relationship that follows from the model by Ostriker et al., which relates the star formation rate to the pressure of the diffuse gas medium. For most of the galaxies considered, there is satisfactory agreement with the model, except for the densest (of the order of several hundred M _?? pc^?2) and least dense (several M?? pc^?2 or less) disk regions.