The structure of giant extragalactic HII regions

Citation

Melnick, Jorge (1977) The structure of giant extragalactic HII regions. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/H0TV-1A14. https://resolver.caltech.edu/CaltechETD:etd-09082008-145630

Abstract

The existence of a close correlation between the linear diameters of giant HII regions in late type spiral and irregular galaxies and the width of their global H[subscript alpha] emission is demonstrated by means of line profiles obtained using interferometric techniques. The relation (diameter-line width) established in members of the local and M81 — NGC 2403 groups is applied to a redetermination of the distance to M101. With the purpose of avoiding the difficulties involved in accurately determining linear diameters of giant HII regions, a correlation between H[subscript beta] luminosity and line profile width has been established. The dust distribution in giant HII regions, which is the factor limiting the possible application of the (H[subscript beta] luminosity-line width) relation to the determination of distances, is studied in considerable detail. It is found that most of the extinction affecting the nebular light comes from dust particles that are mixed with the ionized gas, and that the wavelength dependence of the dust absorption is similar to, but significantly different from Whitford's extinction law. A dynamical model for giant HII regions is constructed on the basis of a detailed study of the 30 Doradus nebula in the LMC. It is shown that giant HII regions can be regarded as aggregates or "clusters" of small HII regions surrounding the hundreds of 0-B stars which ionize the giant nebulae. The highly supersonic velocities observed in these giant HII complexes, therefore, are interpreted as reflecting the motions of the individual stars imbedded therein. It is also shown that galactic shear, amplified during the gravitational collapse of the interstellar cloud out of which the HII regions were formed, can account for at least part of the observed motions.

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