On the effective turbulence driving mode of molecular clouds formed in
  disc galaxies
  We determine the physical properties and turbulence driving mode of molecular
clouds formed in numerical simulations of a Milky Way-type disc galaxy with
parsec-scale resolution. The clouds form through gravitational fragmentation of
the gas, leading to average values for mass, radii and velocity dispersion in
good agreement with observations of Milky Way clouds. The driving parameter (b)
for the turbulence within each cloud is characterised by the ratio of the
density contrast (sigma_rho) to the average Mach number (Mach) within the
cloud, b = sigma_rho/Mach. As shown in previous works, b ~ 1/3 indicates
solenoidal (divergence-free) driving and b ~ 1 indicates compressive
(curl-free) driving. We find that the average b value of all the clouds formed
in the simulations has a lower limit of b > 0.2. Importantly, we find that b
has a broad distribution, covering values from purely solenoidal to purely
compressive driving. Tracking the evolution of individual clouds reveals that
the b value for each cloud does not vary significantly over their lifetime.
Finally, we perform a resolution study with minimum cell sizes of 8, 4, 2 and 1
pc and find that the average b value increases with increasing resolution.
Therefore, we conclude that our measured b values are strictly lower limits and
that a resolution better than 1 pc is required for convergence. However,
regardless of the resolution, we find that b varies by factors of a few in all
cases, which means that the effective driving mode alters significantly from
cloud to cloud.
