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 / \rho _ { 0 } } ) to the average Mach number ( \mathcal { M } ) within the cloud , b = \sigma _ { \rho / \rho _ { 0 } } / \mathcal { M } .
As shown in previous works , b \sim 1 / 3 indicates solenoidal ( divergence-free ) driving and b \sim 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 \mathrm { 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 \mathrm { 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 .