2011ApJ...737...40H

We present results of ¹²CO (1-0) and ¹³CO (1-0) observations of the northeastern spiral arm segment of IC 342 with a ∼50 pc resolution carried out with the Nobeyama Millimeter Array. Zero-spacing components were recovered by combining with existing data taken with the Nobeyama 45 m telescope. The principal 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 kpcx1.5 kpc region located ∼2 kpc away from the galactic center, where a giant molecular association is located at the trailing side and associated star-forming regions at the 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 linewidth-size relation of Galactic GMCs. The identified GMCs were divided into two categories according to whether they are associated with star formation activity or not. Comparison between the two categories indicated that the active GMCs are more massive, have smaller line width, and are closer to virial equilibrium than the quiescent GMCs. These variations of the GMCs' properties suggest that dissipation of excess kinetic energy of GMCs is a required condition for the onset of massive star formation.