Aperture Synthesis of Interstellar Neutral Hydrogen in Absorption

Citation

Greisen, Eric Winslow (1973) Aperture Synthesis of Interstellar Neutral Hydrogen in Absorption. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/3A2K-C664. https://resolver.caltech.edu/CaltechETD:etd-08252008-150426

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The Owens Valley Radio Observatory interferometer has been used to carry out a program of aperture synthesis in the interstellar neutral hydrogen line. Maps having resolutions of 1 KHz and about 1.3 arc minutes have been produced using a novel Fourier inversion procedure which is discussed in some detail. The equipment, observing procedure, noise, and gaussian analysis of spectra are also treated. The Perseus arm feature in the spectrum of Cygnus A is of low optical depth but reveals considerable spatial structure. The data suggest that the hydrogen is located in a thin shell expanding at 2 km/sec, with a radius around 9.7 parsecs and a total mass of 175 solar masses. The Perseus arm feature in the spectrum of Cassiopeia A contains very large amounts of hydrogen with extremely complicated spatial structure. The results may be explained qualitatively by the two-arm spiral shock model of the Galaxy. The principal component in the spectrum of the Crab Nebula is found to show a considerable velocity gradient across the source and appears to contain several condensations within a large, less dense cloud. The absorption feature in the spectrum of 3C353 appears to have a gradient in the dispersion of one component. If real, this result suggests scale lengths in the interstellar hydrogen as small as 0.14 parsecs. The absorption spectra of W12, 3C147, Cygnus A (local arm), and 3C123 are also discussed briefly. Upper limits to the temperature as low as 50°K are found for some of the spectral components although upper limits between 80 and 160°K are more common. Hydrogen clouds with diameters about 1 parsec and densities on the order of 100 cm[…] are shown to be common. The implications of these results for the two-phase model of the interstellar medium are discussed.

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