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Gas disks of spiral galaxies can be described as clumpy accretion diskswithout a coupling of viscosity to the actual thermal state of the gas. Themodel description of a turbulent disk consisting of emerging and spreadingclumps (Vollmer & Beckert 2003) contains free parameters, which can beconstrained by observations of molecular gas, atomic gas and the star formationrate for individual galaxies. Radial profiles of 18 nearby spiral galaxies fromTHINGS, HERACLES, SINGS, and GALEX data are used to compare the observed starformation efficiency, molecular fraction, and velocity dispersion to the model.The observed radially decreasing velocity dispersion can be reproduced by themodel. In the framework of this model the decrease in the inner disk is due tothe stellar mass distribution which dominates the gravitational potential.Introducing a radial break in the star formation efficiency into the modelimproves the fits significantly. This change in star formation regime isrealized by replacing the free fall time in the prescription of the starformation rate with the molecule formation timescale. Depending on the starformation prescription, the break radius is located near the transition regionbetween the molecular-gas-dominated and atomic-gas-dominated parts of thegalactic disk or closer to the optical radius. It is found that only lessmassive galaxies (log (M (M_solar)) <~ 10) can balance gas loss via starformation by radial gas accretion within the disk. These galaxies can thusaccess their gas reservoirs with large angular momentum. On the other hand, thestar formation of massive galaxies is determined by the external gas massaccretion rate from a putative spherical halo of ionized gas or from satelliteaccretion.

Sustaining star formation rates in spiral galaxies - Supernova-driven turbulent accretion disk models applied to THINGS galaxies