Giant Molecular Clouds in the Spiral Arm of IC 342
  We present results of 12CO (1--0) and 13CO (1--0) observations of the
northeastern spiral arm segment of IC 342 with a ~50pc resolution carried out
with the Nobeyama Millimeter Array. Zero-spacing components were recovered by
combining with the existing data taken with the Nobeyama 45m telescope. The
objective of this study is to investigate the variation of cloud properties
across the spiral arm with a resolution comparable to the size of giant
molecular clouds (GMCs). The observations cover a 1 kpc times 1.5 kpc region
located ~2 kpc away from the galactic center, where a giant molecular
association is located at trailing side and associated star forming regions at
leading side. The spiral arm segment was resolved into a number of clouds whose
size, temperature and surface mass density are comparable to typical GMCs in
the Galaxy. Twenty-six clouds were identified from the combined data cube and
the identified clouds followed the line width-size relation of the Galactic
GMCs. The identified GMCs were divided into two categories according to whether
they are associated with star formation activity or not. Comparison between
both categories indicated that the active GMCs are more massive, have smaller
line width, and are closer to virial equilibrium compared to the quiescent
GMCs. These variations of the GMC properties suggest that dissipation of excess
kinetic energy of GMC is a required condition for the onset of massive star
formation.
