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Spiral Types
Published online by Cambridge University Press: 03 August 2017
Abstract
Spiral structure in galaxies and its relation to interstellar gas are reviewed. The evidence for spiral modes and resonance features in two-arm grand design galaxies is summarized. Gas observations of the corotation regions of these galaxies are needed. Multiple-arm galaxies could have several modes simultaneously, or they could lack modes altogether because of insufficient wave reflection in the central regions; then the spirals could result from transient shear instabilities. Flocculent galaxies may have similar shear instabilities but only in the gas. The importance of variable boundary conditions for clouds that flow through spirals is emphasized. These boundary conditions, such as pressure, radiation field, tidal force, and so on, affect the internal structure and molecular fraction of a cloud, and they could influence the calibration coefficient for determining mass from the CO luminosity. The importance of gas observations beyond the optical disk is also emphasized, to elucidate the role of the outer Lindblad resonance on wave propagation, and to study the proposed low-density cutoff for star formation.
- Type
- IV- Models of Spiral Structure
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- Copyright
- Copyright © Kluwer 1991