Star Formation in the Low Surface Brightness Galaxies: a Comparison with the Outer Regions of Normal Galaxies

Abstract: Low surface brightness galaxies (LSB-galaxies) and the outer regions of disks of "normal" surface brightness galaxies (HSB-galaxies) are compared in terms of the radial distributions of star formation rates and efficiencies, as well as the Toomre stability parameter $Q_T$, using data from several samples of galaxies with high $HI$ content. It is shown that the dependences of the star formation rate $SFR$ on the $HI$ surface density and the star formation efficiency $SFE$ on the disk optical brightness $μ_g$ for LSB- and HSB-galaxies have a similar character, suggesting similarity in the conditions that determine the star formation rate in low gas density regions. The considered dependences persist out to distances corresponding to brightnesses of 26-27 mag/arcsec$<sup>2$, where the expected volume density of the equilibrium gas layer should be very low. It is suggested that the disk density at large distances from the center may be significantly underestimated.