Structural model of molecular cloud complexes: Mass, size, and external pressure

We investigate the structure of the molecular cloud complexes (MCCs) as a group of several giant molecular clouds (GMCs) in the Galaxy. Then, we find that the mass–size relation which has been reported for the GMCs establishes well even for the very large MCCs whose size is about 1 kpc. Since the horizontal size of the MCCs is more than the thickness of the Galactic disk, we can no longer consider the MCCs to be a sphere. Thus, we construct a structural model of the MCCs, adopting a rectangular-solid geometry. As a result, our model explains the observed mass–size relation of the MCCs very well. From the estimated external pressure around the MCCs, we find that they are in a rough pressure balance with the interstellar medium. Moreover, we find there is observational deficiency of the MCCs with a large size and surface density. Then, we suggest that the external pressure has a significant effect on the structure and evolution of the MCCs. We also discuss the effect of H ii regions in the MCCs.