Aperture synthesis observations of neutral hydrogen in three galaxies

Citation

Shostak, G. Seth (1972) Aperture synthesis observations of neutral hydrogen in three galaxies. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/FRRJ-YB73. https://resolver.caltech.edu/CaltechETD:etd-08212008-102427

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Aperture synthesis techniques have been used to study the three galaxies NGC2403 (Scd), NGC4236 (SBdm), and IC10 (Irr) in the 21-cm line of neutral hydrogen and off-line continuum. Spatial resolution was 2 minutes of arc; velocity resolution was 21 km-s[...]. The data yield information on the distribution of the HI, and its associated velocity field. Hydrogen extent of the objects agrees well with their optical size, and evidences a lumpy structure. HI peaks occur away from the nuclei for NGC2403 and NGC4236, and are suggestive of a ring structure for the former galaxy. The hydrogen distribution of IC10 is similar in appearance to that of the Small Magellanic Cloud. In all objects HI peaks show good positional agreement with major HII regions. Velocity fields are derived for all objects, and rotation curves obtained for NGC2403 and NGC4236. These curves evidence differential rotation but do not include Keplerian tails, implying that the mass distributions are considerably extended and the total masses poorly determined. The velocity field of IC10 suggests rotation. Non-circular motions of ~10 km-s[...] are seen in NGC2403, and these agree well with the streaming motions predicted by the density-wave theory of spiral structure. A comparison of NGC2403 with the late-type spirals M33 and M101 reveals a progressive deepening of the central HI depression with increasing mass. Rotation curves for these three galaxies are similar, and consequently all are expected to have mass-to-light ratios which increase rapidly with radius. All have similar maximum HI surface densities (~10[...] atoms-cm[...]) and observed fractional hydrogen masses (~9%).

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