The Star-Formation Efficiency and Density of the Disks of Spiral Galaxies

Abstract

For four well studied spiral galaxies (M33, M81, M100 and M101) we consider the dependencies of star formation rate (SFR) and star formation efficiency (SFE = SFR/M_{gas}) both on the radial distance R and on some kinematic parameters of galactic discs. To estimate SFR(R) we used a combined UV+FIR method based on the UV profiles corrected for the interstellar extinction presented by Boissier et al, 2004. It is demonstrated that the most tight correlation, similar for all four galaxies we considered, exists between the local SFE and the surface brightness (density) of discs at a given R (beyond their central regions). To account for the observed surface densities of discs in the frame of a simple conservative model of evolution of gas content (a toy model) it is necessary for the local parameter N in the Schmidt law for a disc (SFR \sim \sigma_{gas}^N) not to exceed unit. Only in this case it is possible to reconcile the observed dependencies \sigma_{gas}(R) and SFE(R) assuming a gas accretion, more intense in the inner regions of galaxies.