The Mass-Size Scaling Relation for MW GMCs

Charles Lada

Wednesday December 2nd, 14:45

In this talk I will discuss an investigation of the nature of Larson's mass-size scaling relation for GMCs in the Galaxy recently carried out by Tom Dame and myself. We examined existing CO and infrared extinction surveys of the Galaxy and confirmed the scaling originally suggested by Larson that implied a constant surface density for Galactic GMCs. However, both the scatter and scaling coefficients differ significantly between the CO and extinction derived relations. Close examination of the much larger scatter observed in the CO relations revealed a systematic variation in the coefficient of the mass-size relation with Galactic radius. This radial variation arises from both a radial dependence of X(CO) on metallicity and an increase in the measured surface densities of GMCs within the molecular ring. The latter increase may reflect both an intrinsic increase in cloud surface densities and a systematic upward bias in the measurements due to cloud blending in the molecular ring. The smaller scatter in the extinction derived mass-size relations is a consequence of the smaller Galactic volume sampled by the infrared observations. We conclude that outside the molecular ring the GMC population of the Galaxy can be described to relatively good precision by a constant surface density of 35 Mo/pc^2. Inside the molecular ring GMC surface densities are estimated to be between 35-80 Mo/pc^2, depending on the amount of cloud blending. Finally, we attribute the difference in scaling coefficients between the CO and extinction relations to be due to an underestimate of X(CO) for local GMCs.