HUNT FOR VARIATIONS IN MOLECULAR GAS AND CLOUD PROPERTIES ACROSS THE INNER, NORTHERN GALACTIC DISK

Presumably, the properties of Galactic molecular gas reflect and diagn ose external circumstances and environment. Nonetheless little study o f variations in the properties of this gas across the disk has been ca rried out. Here, we show that the density-sensitive ratio of bulk (CO) -C-13 emissivities (measured in K km s-1 per kpc of path in the galact ic plane) in the J = 2-1 and J = 1-0 transitions does not appear to va ry systematically across the northern Galactic disk interior to the So lar Circle. For material at kinetic temperatures of 10-15 K, the in si tu densities implied by the J = 1-0/J = 2-1 line ratio in (CO)-C-13 ar e 1500-3000 H-2 cm-3, much larger than the mean values which arise fro m conversion of integrated (CO)-C-12 luminosities or virial masses. Th us Galactic GMCs are directly shown to be lumpy, with filling factors of typically 10%, but the density-temperature product in the lumps has no trend with radius across the inner disk. This stands in contrast t o an earlier observation that the ratio of (CO)-C-12/(CO)-C-13 bulk em issivities measured in the J = 1-0 transition changes over the norther n half of the Galactic disk, being lowest at the peak of the molecular ring. We also search for variations in the properties of defined GMCs (chiefly their line width) using two extant cloud samples from galact ic plane surveys. Although these samples differ markedly about what co nstitutes a cloud, they both show a clear decline in mean cloud line w idth at R > 0.8 R0. However, this is probably an artifact of observati onal selection.