The onset of large scale turbulence in the interstellar medium of spiral
  galaxies
  Turbulence is ubiquitous in the interstellar medium (ISM) of the Milky Way
and other spiral galaxies. The energy source for this turbulence has been much
debated with many possible origins proposed. The universality of turbulence,
its reported large-scale driving, and that it occurs also in starless molecular
clouds, challenges models invoking any stellar source. A more general process
is needed to explain the observations. In this work we study the role of
galactic spiral arms. This is accomplished by means of three-dimensional
hydrodynamical simulations which follow the dynamical evolution of interstellar
diffuse clouds (100cm-3) interacting with the gravitational potential field of
the spiral pattern. We find that the tidal effects of the arm's potential on
the cloud result in internal vorticity, fragmentation and hydrodynamical
instabilities. The triggered turbulence result in large-scale driving, on sizes
of the ISM inhomogeneities, i.e. as large as 100pc, and efficiencies in
converting potential energy into turbulence in the range 10 to 25 percent per
arm crossing. This efficiency is much higher than those found in previous
models. The statistics of the turbulence in our simulations are strikingly
similar to the observed power spectrum and Larson scaling relations of
molecular clouds and the general ISM. The dependency found from different
models indicate that the ISM turbulence is mainly related to local spiral arm
properties, such as its mass density and width. This correlation seems in
agreement with recent high angular resolution observations of spiral galaxies,
e.g. M51 and M33.
