Published online by Cambridge University Press: 14 August 2015
The evidence for warps in the gas layers of galaxies is reviewed. Both the 21-cm line intensity distribution on the sky in edge-on systems and the hydrogen velocity fields in other systems indicate that warped gas layers are common, more common than close companions. Hence, warps probably persist for several times 109 years.
The thickness of the gas layer in NGC 891 appears to increase outward, similarly to that in our Galaxy.
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