Physics
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........27h&link_type=abstract
Thesis (PH.D.)--UNIVERSITY OF MARYLAND COLLEGE PARK, 1995.Source: Dissertation Abstracts International, Volume: 57-03, Section:
Physics
Giant Molecular Clouds, Milky Way
Scientific paper
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.) This thesis includes a spectroscopic survey of HCN, CS and CO emission from 19 nearby spiral galaxies; this study shows that there is an appreciable quantity of dense molecular gas in the centers of most spiral galaxies, not just starburst galaxies, and it demonstrates the feasibility of measuring the relatively weak HCN and CS lines with modern millimeter telescopes. I also present a large-scale 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 HCN and CS in the disk of the Milky Way suitable for comparison with extragalactic results. High-resolution imaging of the HCN emission from the centers of six nearby galaxies using the BIMA interferometer reveals that the HCN emission is largely confined to the central kiloparsec of the galaxies. In the Milky Way, in the nearby galaxy NGC 6946, and in NGC 1068, the ratio IHCN/I CO is 5-10 times higher in the centers of the galaxies than in their disks. Furthermore, in the Milky Way, the I_{HCN }/ICO is correlated quantitatively with the hydrostatic pressure calculated from the measured stellar density distribution. I conclude with a detailed study of BIMA observations of CO, HCN, and 13CO in the inner two kiloparsecs of the nearby Seyfert/starburst galaxy NGC 1068; the distribution, kinematics and physical conditions implied by the observations allow a detailed synthesis of the role of the molecular gas in the inner disk of this galaxy.
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