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Large Scale Star Formation: Density Waves, Superassociations and Propagation

Published online by Cambridge University Press:  04 August 2017

Bruce G. Elmegreen
Bruce G. Elmegreen*
Affiliation:
IBM Thomas J. Watson Research Center, P.O. Box 218, Yorktown Heights, N.Y. 10598 USA

Abstract

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The hypothesis that density waves trigger star formation is critically examined. Much of the former evidence in favor of the hypothesis is shown to be inconsistent with modern observations. A comparison between galaxies with and without density waves reveals no significant difference in their star formation rates. A new role for density waves in the context of star formation might be based on four principles: 1. density waves are intrinsically strong, 2. the gas is compressed more than the stars in the wave, 3. star formation follows the gas, with no preferential trigger related to the wave itself, and 4. regions of star formation are larger in the spiral arms than they are between the arms. This new role for density waves is primarily one of organization: the waves place most of the gas in the arms, so most of the star formation is in the arms too. The waves also promote the coagulation of small clouds into large cloud complexes, or superclouds, by what appears to be a combination of collisional agglomeration and large-scale gravitational instabilities. Special regions where density waves do trigger a true excess of star formation are discussed, and possible reasons for the difference between these triggering waves and the more common, organizing, waves are mentioned. Other aspects of large-scale star formation, such as the occurrence of kiloparsec-size regions of activity and kiloparsec-range propagation, are illustrated with numerous examples. The importance of these largest scales to the overall mechanism of star formation in galaxies is emphasized.

Type
II. Large Scale Processes of Star Formation
Information
Symposium - International Astronomical Union , Volume 115: Star Forming Regions , 1987 , pp. 457 - 481
Copyright
Copyright © Reidel 1987 

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