Sustaining star formation rates in spiral galaxies - Supernova-driven
  turbulent accretion disk models applied to THINGS galaxies
  Gas disks of spiral galaxies can be described as clumpy accretion disks
without a coupling of viscosity to the actual thermal state of the gas. The
model description of a turbulent disk consisting of emerging and spreading
clumps (Vollmer & Beckert 2003) contains free parameters, which can be
constrained by observations of molecular gas, atomic gas and the star formation
rate for individual galaxies. Radial profiles of 18 nearby spiral galaxies from
THINGS, HERACLES, SINGS, and GALEX data are used to compare the observed star
formation efficiency, molecular fraction, and velocity dispersion to the model.
The observed radially decreasing velocity dispersion can be reproduced by the
model. In the framework of this model the decrease in the inner disk is due to
the stellar mass distribution which dominates the gravitational potential.
Introducing a radial break in the star formation efficiency into the model
improves the fits significantly. This change in star formation regime is
realized by replacing the free fall time in the prescription of the star
formation rate with the molecule formation timescale. Depending on the star
formation prescription, the break radius is located near the transition region
between the molecular-gas-dominated and atomic-gas-dominated parts of the
galactic disk or closer to the optical radius. It is found that only less
massive galaxies (log (M (M_solar)) <~ 10) can balance gas loss via star
formation by radial gas accretion within the disk. These galaxies can thus
access their gas reservoirs with large angular momentum. On the other hand, the
star formation of massive galaxies is determined by the external gas mass
accretion rate from a putative spherical halo of ionized gas or from satellite
accretion.
