AMBIGUITIES IN THE IDENTIFICATION OF GIANT MOLECULAR CLOUD COMPLEXES FROM LONGITUDE-VELOCITY DIAGRAMS

Model galactic disks generated from N-body cloud-particle simulations are used to investigate techniques which use longitude-velocity diagrams to identify molecular cloud complexes in the disk of the Galaxy. A procedure similar to the method used to reduce the low-level emission in Galactic l-upsilon diagrams is used to isolate complexes of emission in the model l-upsilon diagram (LVCs) from the "background" clouds. The LVCs produced in this manner yield a size-line-width relationship with a slope of alpha = 0.58 and a mass spectrum with a slope of gamma = -1.55, consistent with Galactic observations. While the procedure succeeds in isolating complexes in the l-upsilon diagram, it does not succeed in unambiguously identifying real cloud complexes in the model galactic disk. We demonstrate that associations identified as LVCs are often chance superpositions of clouds spread out along the line of sight in the disk of the model system. This indicates that the longitude-velocity diagram cannot be used to unambiguously determine the location of molecular cloud complexes in the model galactic disk. The results of the modeling also indicate that the existence of a size-line-width relationship is not a reliable indicator of the physical nature of cloud complexes, in particular whether the complexes are gravitationally bound objects. These results are consistent with previous findings that gravitationally driven orbit crowding, without artificially prescribed cloud coalescence, is sufficient to reproduce the observed distribution of GMCs in the arms of spiral galaxies. Therefore, it could be that substantially fewer GMCs than previously thought are gravitationally bound objects.