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Opaque Spiral Disks: Some Empirical Facts and Consequences

Published online by Cambridge University Press:  03 August 2017

Edwin A. Valentijn
Edwin A. Valentijn*
Affiliation:
European Southern Observatory, Karl-Schwarzschild-Str. 2 D - 8046 Garching bei München

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A recent analysis of the surface brightness profiles of a complete sample of 9381 spiral galaxies extracted from ESO-LV (Lauberts and Valentijn, 1989) showed that many galaxy disks of especially Sb–Sc galaxies are opaque (Valentijn, 1990). This paper studied how the observed surface brightness μobs varies as a function of the observed axial ratio a/b, by fitting the data of samples of spirals with (1) assuming the a/b to give the inclination angle of the disks to the line of sight. A sample of galaxies that were fully transparent at the particular radius used for the test would have C ≃ 1, while C <~ 0.25 signifies opaque conditions, the transition value being heavily dependent on the spatial distribution of the absorbing material and the effect of multiple scatterings (Bruzual et al. 1988). The most frequently-used C values range from 0.5–0.9 (Holmberg 1975, de Vaucouleurs et al. 1976, Sandage and Tammann 1981). However, Valentijn (1990) derived C values well below 0.25 for large samples of spirals throughout the galaxy disks; this result is not significantly affected by selection effects in either magnitude, angular diameter or axial ratio, nor by the presence of bulges (at least for types Sb and later).

Type
II. The Disk-Halo Interface in Other Galaxies
Information
Symposium - International Astronomical Union , Volume 144: The Interstellar Disk-Halo Connection in Galaxies , 1991 , pp. 245 - 256
Copyright
Copyright © Kluwer 1991 

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