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The Rotation Curves of Galaxies

Published online by Cambridge University Press:  14 August 2015

M. S. Roberts
M. S. Roberts*
Affiliation:
National Radio Astronomy Observatory∗, Green Bank, W. Va., U.S.A.

Abstract

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Currently available data on rotation curves are reviewed. For curves derived from optical measurements the distribution of the ratios: the last measured point on a rotation curve to the optical radius of the galaxy has a median value of if Reference Catalogue radii are used and if Holmberg radii are used. It is the absence of easily measurable H II regions that so severely limits the extent of these rotation curves. Accordingly, little can be said of the dependence of Vc on R for large R, where R is comparable to a Holmberg radius. The assumption that a rotation curve approaches a Keplerian curve after passing its peak rotational velocity implies a strongly concentrated and limited extent of the mass distribution within a galaxy. This assumption is not supported by 21-cm observations of the velocity field within a galaxy. Because of the greater extent of H I compared to measurable optical (blue) surface brightness, rotation curves may be defined to much larger radii from 21-cm observations. The median value of the above ratio for 14 galaxies is 1.3. At least 7 of these galaxies show an essentially constant rotational velocity at large R, while 5 galaxies have a slowly decreasing Vc(R). For both types of curves, a significant surface mass density at large R is required, and a large (≳ 100) mass-to-luminosity ratio is indicated. Such values are consistent with a late dwarf M star population (the most common type of star in the solar neighborhood) in the outer regions of a galaxy.

Type
Part II/Flattened Systems
Information
Symposium - International Astronomical Union , Volume 69: Dynamics of Stellar Systems , 1975 , pp. 331 - 340
Copyright
Copyright © Reidel 1975 

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