Other
Scientific paper
Sep 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992phdt.........2s&link_type=abstract
Ph.D. Thesis Wisconsin Univ., Madison.
Other
1
Disk Galaxies, Gas Flow, Gas Path Analysis, Halos, High Temperature Gases, Ionic Collisions, Ionized Gases, Neutral Gases, Spectroscopy, Calcium, Carbon, Explorer Satellites, Ion Distribution, Ionization, Iue, Nitrogen, Silicon, Sodium
Scientific paper
The distribution and physical properties of low density gas in the Galactic disk and halo are discussed. The two major topics addressed are (1) issues related to the formation of highly ionized gas in the Galaxy, and (2) the distribution of neutral gas along low density interarm and Galactic center sight lines. It is anticipated that the results will help to constrain current theories of gas energetics and kinematics associated with phenomena such as Galactic fountains or large scale gas flows. Highly ionized gas is studied via International Ultraviolet Explorer satellite observations of the Si IV, C IV, and N V doublets. The detection of N V indicates that hot (T is approximately 2 x 104 deg K) gas exists along the sight lines. Of the species detectable with operational space-based satellites, it provides the best diagnostic of collisionally ionized gas in the interstellar medium (ISM) because it is extremely difficult to produce by photoionization. The N V profiles are compared to the C IV and Si IV profiles along individual sight lines. A trend for increasing line width as the ionization level increases is found. In many cases, the profile widths are broader than expected from thermal line broadening alone, suggesting that large scale flows of hot gas may exist along some of the sight lines. Portions of the high ion profiles agree with each other, indicating that all three ions may be formed in collisionally ionized gas in some regions. The high ion profiles have very different shapes than the Al III profiles. Al III is a tracer of photoionized gas. The general results of a survey of Na I and Ca II absorption over 57 extended low density paths in the Galactic disk and halo are described. It is found that Na I and Ca II trace different phases of the ISM along the sight lines. Na I traces a cloudy medium, whereas Ca II traces both the clouds and a more smoothly distributed intercloud medium.
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