Simulations of the star-forming molecular gas in an interacting M51-like
  galaxy: cloud population statistics
  To investigate how molecular clouds react to different environmental
conditions at a galactic scale, we present a catalogue of giant molecular
clouds resolved down to masses of $\sim 10$~M$_{\odot}$ from a simulation of
the entire disc of an interacting M51-like galaxy and a comparable isolated
galaxy. Our model includes time-dependent gas chemistry, sink particles for
star formation and supernova feedback, meaning we are not reliant on star
formation recipes based on threshold densities and can follow the physics of
the cold molecular phase. We extract giant molecular clouds at a given timestep
of the simulations and analyse their properties. In the disc of our simulated
galaxies, spiral arms seem to act merely as snowplows, gathering gas and clouds
without dramatically affecting their properties. In the centre of the galaxy,
on the other hand, environmental conditions lead to larger, more massive
clouds. While the galaxy interaction has little effect on cloud masses and
sizes, it does promote the formation of counter-rotating clouds. We find that
the identified clouds seem to be largely gravitationally unbound at first
glance, but a closer analysis of the hierarchical structure of the molecular
interstellar medium shows that there is a large range of virial parameters with
a smooth transition from unbound to mostly bound for the densest structures.
The common observation that clouds appear to be virialised entities may
therefore be due to CO bright emission highlighting a specific level in this
hierarchical binding sequence. The small fraction of gravitationally bound
structures found suggests that low galactic star formation efficiencies may be
set by the process of cloud formation and initial collapse.
