Fast cloud-cloud collisions in a strongly barred galaxy: Suppression of
  massive star formation
  Recent galaxy observations show that star formation activity changes
depending on galactic environments. In order to understand the diversity of
galactic-scale star formation, it is crucial to understand the formation and
evolution of giant molecular clouds in an extreme environment. We focus on
observational evidence that bars in strongly barred galaxies lack massive stars
even though quantities of molecular gas are sufficient to form stars. In this
paper, we present a hydrodynamical simulation of a strongly barred galaxy,
using a stellar potential which is taken from observational results of NGC1300,
and we compare cloud properties between different galactic environments: bar,
bar-end and spiral arms. We find that the mean of cloud's virial parameter is
~1 and that there is no environmental dependence, indicating that the
gravitationally-bound state of a cloud is not behind the observational evidence
of the lack of massive stars in strong bars. Instead, we focus on cloud-cloud
collisions, which have been proposed as a triggering mechanism for massive star
formation. We find that the collision speed in the bar is faster than those in
the other regions. We examine the collision frequency using clouds' kinematics
and conclude that the fast collisions in the bar could originate from
random-like motion of clouds due to elliptical gas orbits shifted by the bar
potential. These results suggest that the observed regions of lack of active
star-formation in the strong bar originate from the fast cloud-cloud
collisions, which are inefficient in forming massive stars, due to the
galactic-scale violent gas motion.
