Eventful Evolution of Giant Molecular Clouds in Dynamically Evolving
  Spiral Arms
  The formation and evolution of giant molecular clouds (GMCs) in spiral
galaxies have been investigated in the traditional framework of the combined
quasi-stationary density wave and galactic shock model. However, our
understanding of the dynamics of spiral arms is changing from the traditional
spiral model to a dynamically evolving spiral model. In this study, we
investigate the structure and evolution of GMCs in a dynamically evolving
spiral arm using a three-dimensional N-body/hydrodynamic simulation of a barred
spiral galaxy at parsec-scale resolution. This simulation incorporated
self-gravity, molecular hydrogen formation, radiative cooling, heating due to
interstellar far-ultraviolet radiation, and stellar feedback by both HII
regions and Type-II supernovae. In contrast to a simple expectation based on
the traditional spiral model, the GMCs exhibited no systematic evolutionary
sequence across the spiral arm. Our simulation showed that the GMCs behaved as
highly dynamic objects with eventful lives involving collisional build-up,
collision-induced star formation, and destruction via stellar feedback. The GMC
lifetimes were predicted to be short, only a few tens of millions years. We
also found that, at least at the resolutions and with the feedback models used
in this study, most of the GMCs without HII regions were collapsing, but half
of the GMCs with HII regions were expanding owing to the HII-region feedback
from stars within them. Our results support the dynamic and feedback-regulated
GMC evolution scenario. Although the simulated GMCs were converging rather than
virial equilibrium, they followed the observed scaling relationship well. We
also analysed the effects of galactic tides and external pressure on GMC
evolution and suggested that GMCs cannot be regarded as isolated systems since
their evolution in disc galaxies is complicated because of these environmental
effects.
