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The goal of this thesis is to investigate the properties of giant molecular clouds (GMCs) as a function of their location within a galaxy: do GMCs within the high-pressure environments of galactic bulges have the same physical properties as GMCs in the disks of galaxies? I use as a qualitative indicator of pressure the ratios of the 3 mm emission from the dense gas tracers HCN and CS relative to that of CO. (HCN and CS typically trace gas densities of 10$^5$ cm$^{-3}$ or higher; this is about 100 times the critical density required to excite CO.) The observations for my thesis fall into three main categories:
(1) A single-dish survey of HCN, CS and CO in the plane of the Milky Way and over half a dozen nearby GMCs. These observations provide for the first time a large-scale sample of the disk of the Milky Way suitable for comparison with extragalactic results.
(2) A single-dish spectroscopic survey of HCN, CS and CO emission from the bulges of 19 nearby spiral galaxies (Helfer \& Blitz 1993). The ratios of integrated intensity I(HCN)/I(C0) are typically 0.02--0.06 over kpc scales in galactic bulges, while the upper limit of I(HCN)/I(C0) on 0.1 kpc scales over solar neighborhood GMCs is about 0.01.
(3) High-resolution imaging of the HCN and CO emission from the bulge regions of 4 relatively ``normal'' nearby galaxies as well as the unusual Seyfert/starburst hybrid NGC 1068 using the BIMA interferometer. The interferometer resolves structures in external galaxies on size scales of individual GMCs or small associations of GMCs; we can therefore measure the radial dependence of I(HCN)/I(CO). In NGC 1068, the I(HCN)/I(CO) is 0.4 in the nucleus and falls monotonically to about 0.04 in the kpc-scale starburst region.
These Galactic and extragalactic observations demonstrate that the bulges of galaxies contain fractionally more dense gas than do the disks of galaxies; it is therefore misleading to compare results from the centers of external galaxies to conditions in nearby star-forming molecular clouds. I discuss some of the implications of this study and its relevance to understanding the role of molecular gas in normal as well as active galaxies.
This work is supported by a grant from the National Science Foundation.
