Other
Scientific paper
Dec 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997phdt.........6y&link_type=abstract
Thesis (PHD). UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN , Source DAI-B 58/06, p. 3093, Dec 1997, 167 pages.
Other
Local Group, Star Formation
Scientific paper
This thesis presents a study of the properties and kinematics of the neutral interstellar medium in a small sample of nearby dwarf galaxies. The selected galaxies include two dwarf irregulars, three dwarf ellipticals, and two intermediate irregular/spheroidal galaxies, most of which are at distances of ~1 Mpc or less. HI synthesis images of these galaxies probe their atomic media at resolutions of 60-160 pc. Observations of CO emission in two of the dwarf ellipticals clarify the relationship between atomic and molecular gas and constrain the physical properties of molecular clouds in those galaxies. Hα images of the two dwarf ellipticals show one probable supernova remnant in NGC 185 but no other HII regions in either galaxy. Of the five galaxies with detected HI emission, the four with current or recent star formation show evidence of a cold atomic gas component. For example, the HI spectra in the irregulars Leo A and Sag DIG can be decomposed into high-dispersion and low-dispersion components which are probably analogous to Galactic cold and warm neutral phases. Twenty to thirty percent of the HI in those irregulars is in the cold component, with the remaining HI in the warm component. The dwarf elliptical galaxies NGC 185 and NGC 205 also show low-dispersion HI clumps associated with molecular clouds and dust patches. LCS 3, the galaxy with the least amount of current star formation, shows only a high-dispersion, probably warm HI phase. These results suggest that a cold atomic component is necessary for, or at least commonly associated with, star formation. The column densities of atomic gas around the molecular clouds in the dwarf ellipticals, as well as the presence of cold and warm HI phases in the dwarf irregulars, are explained by current models of the interstellar medium. The apparent lack of a low-dispersion, cold HI phase in LGS 3 implies that the thermal pressure is too low for a cold phase to be stable. Only two of the five galaxies with HI emission show good evidence for rotational support. Non-circular gas motions on the order of 10 km s-1 are significant sources of support against gravity for all of the five except perhaps NGC 205. These non-circular motions may even be the dominant sources of support against gravity, in contrast to the situation in more luminous irregular galaxies and spirals. Estimated mass-to-light ratios vary from 8-10 for the dwarf ellipticals NGC 185 and NGC 205 to something less than 2-3 for the dwarf irregular Leo A; thus, these results confirm the large dispersion found in mass-to-light ratios of other low mass galaxies. The distribution and kinematics of HI in the dwarf ellipticals NGC 185 and NGC 205 are significantly different from those in giant ellipticals, so that an internal origin for the gas in the dwarf ellipticals should be considered. Finally, the presence of gas in LGS 3 conflicts with evolutionary models which predict that a burst of star formation activity should have removed all the gas from that galaxy.
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