| Total IRAS fluxes at 12 mum, 25 mum, 60 mum, and 100 mum for eight Giant Molecular Clouds (GMCs), W3, NGC 7538, W51, S235, GemOB1, S 155, S 140and G216--2.5, have been computed from the IRAS Sky Survey Atlas maps by accurately subtracting the large scale background emission from the Galactic Plane. Total integrated intensities in the 12CO and 13CO emission lines for both clouds are computed from large-scale maps obtained from the literature. Infrared luminosities, Lir and molecular gas masses, M(H2), are computed on 4 x 4pc scales for all clouds, and the ratio Lir/M(H2) is used to characterize the nature of the heating sources throughout them. The total molecular gas masses range from a few 104 -- 3 x 106 M⊙ , encapsulating nearly the complete range of GMCs masses seen in the galactic disk, and showing typical diameters and linewidths. We find evidence that two of the clouds, GemOB1 and G216-2.5, may not be in virial equilibrium, consistent with previous studies. The remaining clouds appear to be in approximate equipartition of their kinetic and gravitational energy.; The total infrared luminosities, and the observed cloud-averaged Lir/M(H2) ratios, are within the range of inner-galaxy GMCs. There is no clear dependence of global Lir/M(H2) ratio with cloud mass. However, we find in GMCs the global cloud averages are not representative of the actual distribution of Lir/ M(H2) values observed throughout each cloud, and therefore should not be used to constrain the star formation properties within clouds. The bulk of the luminosity output from GMCs appears to be produced within a small fraction of the total cloud mass, and that fraction increases with total cloud mass. The bulk of these clouds by mass and volume is relatively "quiescent", with low mean Lir/ M(H2) ratios. Regions of two of the clouds have extremely low values, and have characteristic temperatures consistent with primarily heating by cosmic rays. The quiescent regions of the other clouds have higher characteristic temperatures indicative of substantial additional heating, and these ratios appear to increase with the luminosity of the active region in the same cloud. Heating throughout the bulk of the cloud volume is likely to be due to either direct and reprocessed radiation from the active cores, or additional distributed low-mass star formation throughout the cloud volume, or both. The present data are insufficient to distinguish between the two mechanisms. Observational strategies for characterizing the range of physical properties and star-forming activity within GMCs are discussed. |
